I think it is probably just a tradition ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯. It's for children, it stars children. It doesn't have to, but that's just the way it is. I sort of mostly don't buy the 'anime is for adults' I think that is just marketing and suits a western adult babies funkopop market. (I do know there are exceptions of more interesting animes with complex themes, but it seems like the main bulk of animes are actually for children - even when they are 'dark').

But I must once again state I'm no expert lol.