Those kids are too mature, they talk and behave too much like adults, it feels weird.

that's the case for most kids of nobles in manga and anime, well from the ones ive seen

It's interesting you say that cuz in most isekais(shoujo) that have this premise the MCs don't act mature or bring ideas from their own world. It's funny since the MC in Daughter of the Emperor is an adult in a child's body but acts like a kid who wasn't reincarnated(thrown out the window). Also uses her knowledge(of our world) on nothing, it was interesting but then became some sort of sitcom.
*cue seinfield music* ╮(￣▽￣)╭