Help with zippered Pajamas?

My 2 yr old Granddaughter has several footed, zip up pj's. She has started unzipping them and taking them off in her bed. Please give me some ideas as how to keep her from getting to the zipper. Like maybe sewing in a snap to go over it, or something like that. I don't want to use a safety pin because I am afraid it might come undone and stick her.