Adventure Time is the kind of animated TV-show that plays to the particular strengths of the cartoon format. If you’re looking for realistic characters or serious stories, this is obviously the wrong address. If you looking for something completely zany and off-the-wall, this is perfect though. Like most cartoons the world in Adventure Time only relies on physics when its suits the shows goals, and eschews them when it’s the right thing to do (which happens more often than not).
In that regard, it’s actually a better fantasy than what is (in prose fiction at least) usually associated with fantasy. This is not a world with magic and supernatural beings bound by a rule set that rivals hard SF in its rigidity, but a world whose topology and general rules change on a whim. It is stylistically consistent, but doesn’t really make sense when it comes to stuff like biology or physics or geography (things are always as near or far away as the plot demands).
If you would ask me whether this is a kids shows, I probably had a hard time answering. It looks like one, but I think the humor is often more geared toward grown-ups whose inner child is still alive, than real kids. But like so often, even when they don’t get everything about the show, it’s wacky nature should satisfy most kids anyway. One of the best animated shows in recent time.