Well, whatever happened to 7 year-old children singing "99 Bottles of Beer on the Wall"? We sang that all the time in elementary school on bus trips, enough to make the bus driver rip out their hair. If someone's parents were there, they might occasionally scold you, but then like a smartass you'd just change the words to "99 bottles of ROOT beer on the wall, 99 bottles of ROOT BEER..."
Look through the pop culture references to the song in the Wikipedia entry -- almost nothing after the 1990s. There are two Disney movies from 2000 that allude to it, and another in 2003. (There's an episode of 30 Rock that mentions it, but that isn't even for kids.) There are references to it in a 1963 movie, so Baby Boomers must have heard it growing up, hence its appearance in Seinfeld and National Lampoon's Vacation.
Searching JSTOR, the earliest reference I found is from a 1962 article in the journal Western Folklore, which also cites a 1954 article in the journal in which the song is called "29 Bottles of Beer." The 1962 article doesn't say what the words are, but the 1954 version is different from the one I learned in the '80s which includes the phrase "take one down, pass it around." Back before the crime rate and rock music started soaring in the late '50s, the 1954 version goes:
29 bottles of beer on the wall, 29 bottles of beer.
If one of them bottles should happen to fall,
There'd be 28 bottles of beer on the wall...
So it's more like the British "Ten Green Bottles" song that doesn't have the children singing about passing around the beer. There are no other references to the "29 Bottles of Beer" version, so some innovative Baby Boomer children must have decidedly gone with "99" in the late '50s or early '60s. No other JSTOR references include the lyrics, so I don't know exactly when the song took on the more adult theme of getting drunk, but it was sometime between the mid-'50s and the mid-'80s. The most recent article is from 2000, which includes it among a list of children's songs with well known tunes. Before that is a 1986 fictional work where one character hums the song. So again, it looks like this song has started to die off during the tame times of the later-'90s and 2000s.
In all of my three years I spent working at tutoring centers in the mid-2000s, surrounded by primary and secondary school kids, I never once heard the song, even though students often get bored or restless and find ways to make light of the boredom. The real group to interview is school bus drivers -- they'd surely know if it were less common among kids than it used to be.
I wonder whether we're going to see a shift in generational affiliation from the mode that has been standard from roughly the Silent Generation through today, where generations include no more than 15 years' cohorts, and often just 5 or 6 years' worth (as with the mini generation sandwiched between cultural Baby Boomers, who weren't born after 1956 or '57, and Generation X, who weren't born before 1965). Those differences are real, but an even larger generational gap is becoming apparent between anyone born between roughly 1943 and 1985, and those born after. The former came of age during wild times, and the latter during tame times. That source of environmental influence is a lot stronger than influences that can swing back and forth over a 5 or 10-year period. (The two upward phases of crime waves in the 20th C. lasted around 35 years.)
Everyone in that roughly 40-year cohort heard and spread some version of "99 Bottles of Beer," played some variant on the kissing games "seven minutes in heaven," "truth or dare," "boys chase girls," etc., listened to and re-told an urban legend like "the kidney heist," lived through a new form of rock music growing up, and got to taste French fries that were cooked in beef tallow and popcorn covered in coconut oil instead of the ungodly vegetable oils they've both been cooked in for the past couple decades.
Those born in more recent cohorts have instead had much safer and materially prosperous lives, if less thrilling and fulfilling.