Agreed that children don't have a lot of good reasons to be happy, but not sure that's very relevant here. My observations of children suggest that when they're raised well they have a really high happiness set point, or are just plain better at being happy than adults for developmental reasons unrelated to their circumstances. I don't really trust my own memories of childhood, but the children I hang around always look like they're having more fun than adults nearby, unless they're specifically being prevented from doing so.