I think a key thing for the story of an MG is that the MC should be the problem solver. So whatever adventure he/she has, it needs to be resovled by them, and not sort of deus ex machina with the parents interveening. I think there is a reason why so many books have their MC as an orphan. It isn't done because everyone else does it, but because it means the MC has to take care of himself, and can't go running to a parent for help. Nonetheless, I do think you can have parents in an MG, just make sure they don't spoil any of the fun! They do make an excellent obstacle though.
Don't know much about chapter books however. Anyone else?