This particular party will be my parents, my MIL/FIL, and my BIL/SIL/their three little boys. It's for my 3-year-old's birthday party (we don't usually have "kid" parties until the 4th birthday, and since we just moved here we don't have anybody to invite, anyway). So it's just family, but with BIL/SIL it's slightly more expansive than just us and the grandparents.
Anyway, I took all of your advice and got two tubs of ice cream: chocolate and chocolate-chip cookie dough. While they both have chocolate in a way, I think they're somewhat different from each other and generally much-beloved in our family. I know both grandparents like them, at least, and cookie dough is the birthday girl's favorite.
As to size, they're 1.5 quarts. I don't *think* we'd eat all of them, but if we do, there will probably be other half-eaten ice cream in the freezer I can sneak out. Unless my husband eats all of it before next weekend.