Such a man-child he can be, petulant and stubborn, the casual cruelty, and then there's the insecurity beneath it all. She's too old for him; or rather, he's too young for her. And yet he's so old, too. Too old.
That is just an absolutely wonderful, concise description of Snape. You've summed up so much right there. And it's fabulous that Poppy sees and understands it. Her easy acceptance of his sniping and irascibility is lovely, as is the companionship and comfort and routines they've developed. She's the perfect person to deal with him as well as mediate things between Severus and Tobias. Nice progression of things here; not at all rushed, but perfectly paced.