I think it's they are only a handful good written and acted kids on tv. It's hard to write for kids and the acting isn't as good. Sometimes kids gets used for the plot only to cause drama for the adult characters or kids gets used for comedic relief, but they barely have actual personality. I don't think it has anything to do with acting like kids. The best child on a tv show is in my opinion Sally draper on Mad Men, because while she caused drama between adult characters she still had her own arc and was complex character with personality. I don't think she ever gets any hate. The character and actress gets a lot of praise. It's just really hard to write for kids and casting child actors is definitely hard.