I think everyone wants the experience of living, but it's not a great one without money, and money commonly requires full-time work until financial freedom. But that point usually comes at 65.
So it's not that only children appreciate life, it's just that adults have to deal with that whole money problem, while children don't.
I take a 8 hour work day over being subjected to more or less random weirdness from "adults" every day. IOW, not everyone has this rose-tinted view of their childhood. I still remember adults saying "you will once look back at your childhood and miss the times". Turns out that was totally and utterly wrong. I even have bad dreams about my school/childhood time every few months. I am sooooo glad that time of being subjected to the will of others is over.
I think the problem is that some people do genuinely have a great childhood, or another phase of life (university is another one), while others have a terrible childhood or another phase of life, and people who have one fail to understand those who have the other.
To add to that I think people can have a rose-tinted view and deny what they felt at the time.
I miss the 70's when I was in high school. I had a great time. It's one reason why I drive a '72 Dodge. I feel like a kid in it every time.