Braelyn was just pushing the plate away after that final bite, showing that she was done. Well he would have a general idea of about how much she did eat. If he wasn't wanting to waste food, doing half the amount of what he had gotten, or perhaps just a little bit more than half the amount so if she was feeling a little more hungry.
She wouldn't be able to find anything that she could use to escape; the room was windowless and the door was deadbolted from the outside. It was the room that Damian used for when he was personally training, although that wasn't often at all.