It is definitely allowed with the exceptions noted by PP. That said, we did run into waiters both times my kids shared one (adult) TS entree who made me feel cheap for doing so. (Notable: my kids are young; they shared a giant adult meal; and we ordered each child a dessert and beverage individually.) Both times the restaurants were pretty empty (it was late August).
But, both waiters seemed to be the less fun type compared to the other waiters/waitresses I saw at each restaurant (for example, at 50's Prime Time 2 other waitresses talked to and joked around with one of my kids, but our waiter never spoke to anyone but me and seemed to hate his job). So, maybe it is rare for the waiter to make you feel like you're cheap if you share, and I just lucked out getting 2 of them.