I have large dogs (Bernese Mountain Dogs) but no children. (I did, however, teach middle-schoolers....) Your cousin is doing her child and dog no favors by not training them in positive behavior.
Regardless of your decision on the upcoming visit, it would be a kindness to inquire whether she's aware of "positive reinforcement training" for dogs and, if so, whether she's used it (or can get help) to accustom the dog to either a large crate or an ex-pen--short for "exercise pen," a specific kind of portable enclosure to keep dogs in one place.
Info on positive reinforcement training is all over the Net and in books/magazines, etc.--just don't go anywhere near Cesar Millan, the Dog Abuser. Here are some places to start:
--Whole Dog Journal http://www.whole-dog-journal.com/
--Peaceable Paws http://www.peaceablepaws.com/
And, in case some of the dog's behavior is due to anxiety in new places or around unknown people,
--Fearful Dogs http://fearfuldogs.com/books/
Baby gates may help enforce the boundaries of the house for the child; perhaps letting his mother, your cousin, know that you'll use baby gates to divide the house into "food" and "non-food" zones may help clarify the rules both for her and her child.
I've used baby gates for my dogs, even though they're designed for use with young children. When a new rescue comes to live with us, the baby gates give my cats a safe haven, a room or two where no dog can get close to them. (Berners aren't known as cat-killers or even -chasers, but feline peace of mind is priceless.)
Hope it all works out!