The Tyrannosaurus Rex's Bite attack reads as follows:
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 33 (4d12 + 7) piercing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 17). Until this grapple ends, the target is restrained, and the tyrannosaurus can't bite another target.
Unlike 3.5e there doesn't seem to be any option to try to swallow the creature at this point. But it seems unlikely that the dinosaur would just hold/restrain the victim in its mouth. It says the tyrannosaurus can't bite another target, but can it keep biting the same target?
A logical guess would be that it does another 4d10+7 damage the next turn, without having to roll to hit, since it is already holding the creature between its teeth.
RAW though, I think that the dinosaur would still have to roll to hit, with advantage due to the victim being restrained.
What do you think the answer is RAW, and what would you rule as a DM?