IOWA CITY — Oliver Hanlon was beaming ear-to-ear as he stood next to Senator Elizabeth Warren of Massachusetts on a riverside stage at the University of Iowa.

“I met the president,” he shouted, bounding down the steps two at a time.

His mother gently corrected him: “Not yet,” she said, while the Warren photo line snaked on behind her. Oliver couldn’t be faulted for his civics proficiency; he’s 7 years old.

The 2020 Democratic presidential campaign stops are crawling with children, some of whom spend most of the stump speeches crawling away from their parents. While all of the candidates seek to portray a family-friendly image, it takes work to bring a youngster to a campaign event: They often start late and run well into the evening, with a policy discussion not exactly accessible on the elementary-school level; bigger rallies often take place outdoors, regardless of weather.

Kissing babies and tousling children’s hair has long been a staple of retail campaigning, but in 2020 it is about more than a photo op, even the cutest of photo ops . In a historically crowded field, in an era of uncertain polling, these campaign stops are something of a past-their-bedtimes straw poll, revealing which candidates young parents deem viable or famous enough to risk a tantrum or a sleep-deprived meltdown over the next morning.