After nearly 1 million children were left out of the U.S. census in 2010, the federal government formed a task force to tackle the problem.

But lingering confusion over whether a citizenship question would be added, changes to how the Census Bureau is conducting this survey and a host of other challenges have advocates and experts sounding the alarm about the consequences of an even more inaccurate count in 2020.

"Kids do better when they're counted," Deborah Stein, network director of the Partnership for America’s Children, a Washington-based nonpartisan child policy advocacy coalition, said. "When we count kids, there’s more federal funding for children’s health insurance, for child care, for foster care, for adoption, for schools, for special education, for all the programs that kids need. If we count all our young children, your state and community will get more of that money."

Precisely measuring the number of children in the country has confounded the government for decades. William O’Hare, a former Census Bureau research fellow and an expert on the issue of child undercounts, told NBC News in a recent interview that according to his analysis of census data, the net undercount rate of children has more than tripled since the 1980 survey. O’Hare said that while he couldn’t provide evidence that pointed to any one reason, he believes it’s partly because minority groups that are more likely to be undercounted — Hispanics in particular, as well as Asians — have grown since then.

The bureau's child undercount task force, which began work in 2013, came after statistics revealed that children ages 0-4 were undercounted more than any other age group in the 2010 decennial survey.

Karen Deaver, the task force’s leader, said she calls herself "a Lorax" — a reference to the children's book by Dr. Seuss, in which a character called the Lorax speaks out on behalf of an endangered environment.

"I'm there to make sure that everybody remembers that we want to do our very best to count every young kid," she told NBC News in a recent interview, noting the number of federal programs that are affected when children are undercounted.

To reduce child undercounts in 2020 and beyond, specific language about children and babies was added to solicitation materials, while the language on the question that asks people to list everyone in their household was clarified to remind people to include children and babies.

Other actions include creating kid-focused social media and promotional items, according to an outline Deaver shared with the National Advisory Committee, which focuses more broadly on increasing census participation and reducing undercounts.

But O'Hare said that "negative forces" surrounding the 2020 census, particularly residual fallout from the Trump administration’s now-abandoned push to add a citizenship question to the questionnaire, have the potential to cancel out the bureau's stepped-up efforts to count children, "to the point that we may have the same net undercount in 2020 as we had in 2010."

O'Hare added that "to some extent, that may be a victory, I guess, given the increase we’ve seen over time and given all the adverse issues that the Census Bureau and the census had to deal with this time around."

Children who are most at risk — and what's at stake

Since almost 1 million children were left out in 2010, states have missed out on at least $550 million a year in various federal funds, according to research by Count All Kids, an advocacy campaign mounted by Stein's organization.

Deaver's task force, too, has noted the need to educate communities about the impact of undercounting children, including the implications the problem has for political representation, planning for schools and libraries as well as money for public benefits like Temporary Assistance for Needy Families and the National School Lunch Program.

Certain characteristics increase a child’s risk of not being counted, research shows. Children who have younger parents, for example, are more likely to be left out, as are children living in lower income or limited English speaking households, according to the Census Bureau. O’Hare said other research points to children in either rural areas or densely populated cities to be harder to count, too.

Children who live in "complex households," which the Census Bureau child undercount task force defines as "a household that is not a nuclear family or single-parent family,” or children who do not live with their biological parents, also run a greater risk of being missed.

In 2010, 50 percent or more young black and Hispanic children were reported to be living in complex households, compared to 29.1 percent of young white children, according to the bureau's numbers. More than 63 percent of young Native Hawaiian/Pacific Islander children were living in complex households.

Outside groups get involved

Some advocates cited concerns about testing cutbacks, as well as the bureau's move to conduct the count online, as reasons they think more children could be missed in 2020.

While Deaver and O'Hare were optimistic that the internet could help boost response rates — O'Hare noted that younger children tend to have younger parents who might be more comfortable answering the questions online — other experts felt the risks could outweigh reward.

Count All Kids says the approach is untested. And Douglas Massey, director of Princeton University’s Office of Population Research, noted that among other potential problems, not all people in the U.S. have access to the internet.

“It’s the first census that’s going to be administered over the internet, and there are a lot of things that could go wrong, ranging from Russian hacking to undermine the census to people not being familiar with the internet, who are not having internet connections,” he said. “And this is all in the context of white nationalism that has scared a lot of immigrants, so there are just a lot of things that can happen.”

Meanwhile, groups like the Annie E. Casey Foundation, a Baltimore-based nonprofit that advocates for policies that benefit children, have mounted their own efforts to try and reduce child undercounts by pushing states, county and city governments to come up with strategies to reach the households of young children. Flo Gutierrez, a senior research associate for the foundation, said it has mainly invested in states in the South and southwest — Texas, in particular — but also in New York and New Jersey, places that have a high percentage of communities that are historically hard to count.

"Texas is a state that has a high percent of kids who are living in hard-to-count communities, and they are among the states that had the highest undercount of kids in 2010," she said. "And they’re also a state that hasn’t set aside funding to make the census a priority."

The nonpartisan Urban Institute, in a June report projected as many as 4 million people could be left out in 2020, and warned that Texas, along with California — the two largest states, both with large numbers of immigrants — could face the greatest overall risk, with up to 2 percent of their population going uncounted.

Though President Donald Trump announced in July that he was backing off his effort to include a citizenship question in the 2020 census, immigration groups have said lingering confusion could still depress response rates.

“About 20 percent of the young child population lives in a household with one or more noncitizens,” O’Hare said. “That’s a higher percent than any other age group. So, to the extent that fear reduces the responsiveness of those households, it’s likely to hurt more children more than any age group.”

Deaver said her goal is to communicate to everyone, including parents, that the census is easy, important and safe to complete — and that when children are left out, the next chance to get counted won't be for another 10 years.

“Despite all of our efforts that we do, despite all the messaging that we have around it, some people aren’t going to answer it or they will leave people off their response, and their communities can suffer as a result,” she said. "And when those people that are left off are the children, that's a whole childhood."

Deaver said that while it's unrealistic to expect the census to get everybody, missing children in the count can lead to missing out on federal funds for libraries, parks, housing assistance and medical care. "Maybe there’s not an emergency room until you get to the next county," she said. "All of those things that can affect a child are still important, and so my concern is we’re going to miss one."

One bright spot, O'Hare said, is that both the bureau and outside advocates “have been mobilized to get young children on the census at a much higher degree than they have in the past."

“So that's a positive force,” he said.