It's hard.
Like really hard if you're going for a genuine kid character.
In some stories, the gag is that the kids are like mini-adults (for example, in Peanuts, Charlie Brown and his friends are all a bunch of anxiety-ridden adults in kids' bodies), so that's often the route that people take in writing. It's easier because many of us don't quite remember childhood down to a T. The problem is that Mr. Enter seems to be trying to go for a minor degree of realism in this show in the way the kids act, yet everyone is more of a stereotype or caricature than an actual character.
Just to put in a personal anecdote, I wasn't one of the normal kids. I was obsessed with storyboarding even in preschool. When other kids would point at trucks, I would notice interesting fire hydrants. Now, I'm not saying that every kid has to act like I did, but I'm saying that kids act differently from each other. No two are quite alike.
On another note, I think the best-written kid character I've ever seen was in the "Starkid" musical Trail to Oregon. The son just does random shit like eating all the family's food and throwing his shoe off a covered wagon and when asked why, he replies something along the lines of, "I don't know. I'm seven years old. I don't know why I do anything. I have to experiment with the world."
That just feels so right because kids just do random stuff for the hell of it. Think about it, didn't you just do shit...because? To really write a natural child character, you need to tap back into that time of innocence and mild stupidity.