Curse words in MG seem like a major no no. Granted I'm working off of books I read as a kid and the more recent stuff I read has been animal fantasy, but I've never seen bad language in MG books. The worst it gets is in something like Redwall, where the clear villain characters might drop a "hell" or two. "Shit" would be a big deal.
Though thinking back, I remember being 7 and giggling at the word "toilet."

So it's entirely possible I read really tame stuff.