I can barely even imagine how hard it is to make a good children's movie—that is, a good movie for children. Not a movie for good children, which would be silly.
That's because, as I've suggested, all kids' movies are on some level didactic. They're meant to teach their audience something about The Way The World Is, because the audience is presumably still receptive to stuff like that. Kids don't generally leave theaters initiating sophisticated worldview conversations with their parents, so a lot of the subtle thematic or formal devices you can use with adults—irony, complex themes, really anything that requires extended rumination—is totally off the table. That's not because kids are stupid, but because movies themselves teach kids how to be smart about movies. Kids aren't dumb for not understanding algebra if they don't even know their times tables yet.
So most kids' movies are incessantly entertaining to combat the low attention span of children (perhaps only rivaled by the attention spans of people reading articles online). And their message has to be clear, and correct, but not so overbearing that parents roll their eyes.
To me, this sounds insanely difficult, which is probably why so few movies do it well. Play it too straight, and you risk reinforcing cultural assumptions and being super boring. Try too hard to subvert expectations and you send weird, confusing, deeply problematic messages.
So hopefully you'll understand why The Lego Movie made me so happy, why I left the theaters smiling from ear to ear, texting all my friends to let them know that this was a really, really good movie.
It's not particularly artistic or deep, but it does ...