IMO, whatever theme floats your boat!! I think POP would fun for the grandparents, because the 2 yr old really won't "get it". They will just big, bright, loud colors. Trust me, all my 2 yr old knew was we were at Disney. She called the room "our Mickey Mouse house" and we were in All Star Music.
If you wanted to all be together, the All Star Music has a family suite. There is a bedroom with a QS bed, and the living area has pull outs for a full & 2 twin size beds. You also get 2 bathrooms, 2 tvs, and the kitchenette. It is about $15-20 more a night than getting 2 regular rooms. This is the cheapest option for all of you to be "together" without being on top of one another. You will all fit in a single room at any resort, because your child is an "infant" in Disney's eyes, but that can get crowded only having 1 bathroom for 4 adults.