Since they still have plenty of time before the witches' dinner, Grandmamma decides it would be best to try to get Bruno back to his parents. Bruno agrees, since they're the ones who feed him, after all.
They head down to the lobby, Bruno-mouse and narrator-mouse in Grandmamma's handbag. Bruno is eating. Narrator is peeking out.
Grandmamma finds Mr and Mrs Jenkins and tries to break the news to them, but it doesn't quite go over as planned. First, Mr Jenkins refuses to speak with her in private. Then, when she explains that Bruno "has suffered a rather unfortunate mishap" (16.32) and that he's currently in her handbag, and that, actually, he's a mouse, Mr and Mrs Jenkins pretty much flip out. They don't believe her.
So Grandmamma decides that the proof is in the pudding. She takes Bruno out of her bag and puts him onto a table. Naturally, Mrs Jenkins nearly dies from fright and Mr Jenkins yells at Grandmamma. She surrenders and leaves, taking Bruno (and our narrator) with her.