It's a movie for kids, people! Get in the spirit and watch it like a kid would, and have a little fun.
Okay, the plot's kind of confusing aside from the opposite day premise, and the acting is, well, often juvenile, and it's all a lot of silliness, but that's the point. It's like kids wrote and directed it, which is what makes it good in the eyes of a child.
Of course, the message is that being a parent is harder than it seems -- duh, any adult can see that one coming. But most kids won't.
My 7-year-old girl enjoyed it, and it was fun to watch it with her as her parent -- and that's all that matters.