[Well, the only answer is probably magic, and either it was his mom, Rumple, or a fairy. The look on Henry's face is very much worried, but instead he moves on a little.]

Something kind of happened while you were gone. My mom's fine, but...her mom died. She came back, since that's what happens here. I just know my mom, and she'll pretend like everything's okay, probably.

[Maybe Emma is okay, but Henry knows what it's like when a parent dies, even if they come back. And he's pretty sure Hook cares in a way that means he should know what happened.]