My son and I loved it.
YAY!!!!! YOU WON!!!!!
Now, how to sneak the boy off to see THE DARK KNIGHT.

From everything I've heard, Ledger's portrayal of The Joker is one of the singlemost disturbing things ever. Even I know better than to expose my son to that.
In my opinion, Hellboy is very kid friendly if your kid is into magic and monsters. A couple of red shirts get eaten alive early in the film, but there's no actual gore or blood. I think Hellboy gets a nosebleed at one point and that's about it for blood. It's all very fantastical and cartoonish. A big red demon punching bad guys and monsters with his "Fist of Doom" is one thing. Watching The Joker torture, dismember and set people on fire is another.