I think you're wrong.
Children can't judge looks. If you think of who you found attractive on TV, for example - you may realise that as an older individual your taste has dramatically improved. This is all that has happened with the "pretty children". You may also have mistaken "popular" for "pretty" when you were a kid, and perhaps you are less likely to do that now.
You could look back on school photos and test my theory.
Obviously sometimes a genuinely pretty child will become much less attractive with age, but generally an adult can look at a pretty kid and think "they'll be pretty when they're sixteen" and that's true. Think of scarlett johansson, hayden panettiere, elija wood. They were all adorable creepy little child actors, and they continue to be "adorable" today.