Hmm... well that's a bit tricky. I'd say it's "Parental Guidance Suggested". There is no excessive gore, sexual situations, or cursing, but it can get pretty dark in tone and intense sometimes. Sometimes the tone can get a bit dreary, but it's balanced with humor, too.
Personally, I think it's fine for older kids, though younger ones might be a little freaked out (though there is plenty of silly cat and mouse action).
I tried to make it similar in tone to "creepy" kids movies like Secret of NIMH, Labyrinth, or more recently, something like the recent Jungle Book remake. A bit dark and intense, but mostly fine for kids.