Of course, it varies a lot. THE WHIPPING BOY is about 13K, and it won a Newbery--definitely considered MG, not chapter book. and THE MYSTERIOUS BENEDICT SOCIETY tops out at over 100K, which is monstrously long for MG, but it's so well-paced that it's hard to imagine it any smaller.
To quote former Tricycle books editor Summer Laurie: children's books should be as long as they need to be--and not one word longer.