Cars is one of the least-liked Pixar movies on most lists. Most people agree that even the worst Pixar movie is better than the average film, but beyond that, I think it is very underrated. I think it's one of the best, especially for young kids, for many reasons.
The story is very simple: a racecar wants to win a race, but he gets lost on the way, and winds up making new friends. That's it. It's not a complicated plot. There's no violence, other than cars pushing each other around a bit. Nothing scary. No dead parents. No monsters. No warping of time and space. No evil villains. No giant sharks. No sadistic kids. No toys getting pulled apart or being tossed into a grinder or incinerator. You don't need to know anything about how the world works — fossil fuels, energy production, land rights, or economics. No complicated references to other film genres or mythology.
Granted, Cars 2 is pretty much the exact opposite, but the original Cars has many great qualities that a lot of people overlook.
UPDATE: Antony Johnston was kind enough to let me talk about Cars on his podcast, Unjustly Maligned. The episode is here.