“Kids have really turned into the little conscience sitting in the back seat,” said Julia Bovey, a spokeswoman for the Natural Resources Defense Council, a leading environmental group that recently worked with Nickelodeon on a series of public service announcements and other programming called “Big Green Help.”

“One of the fascinating things about children is that they don’t separate what you are doing from what you should be doing,” Ms. Bovey said. “Here’s this information about how we can help the environment, and kids are not able to rationalize it away the way that adults do.”

In Clinton Hill, Brooklyn, Jan Schmidt, a stay-at-home mother, and Mark Goetz, a professor of furniture design, have watched, amazed, as their 4-year-old son chastises them for letting the water run while they brush their teeth. “He’ll come over and turn it off and say, ‘Every day is Earth Day,’ ” Ms. Schmidt said. “He learned it at school.”

Their older child, 12-year-old Elly, extols the clothesline in her bedroom the way other girls her age might show off a new beanbag chair. An aspiring marine biologist, Elly raised $250 last year to protect coral reefs by selling handmade earrings at school. And she was a big factor in the family’s decision to hang on to their current car instead of buying a bigger one.

“With Elly, there’s sort of an unspoken thing about not buying an S.U.V.,” Ms. Schmidt said.

Elly elaborated: “I wouldn’t be happy if they bought an S.U.V. because they’re not fuel efficient, and they pollute more than other cars.”