What do you do if your grandchildren misbehave?

I have a 5 year old granddaughter that refuses to follow even the basic rules. an example is jumping on the couch and bed.she won't wear her seat belt. she wants you to make her something to eat and when you do she either say's she doesn't like it or changes her mind and wants something else. i gave her a doll as a gift and when she opened it,she said she hated it. she tell's me i'm not the boss of her and i can't tell her what to do. i have never dealt with this kind of behavior. suggestion's please?