Have you asked her/them how she/they would like you to respond? That's probably a good first step because there's no universal way that everyone with DID wants people to respond when they switch. That can vary based on the system, which parts are out, and their relationship with that particular person.

Personally, when my child parts are out, I find it most helpful when people interact with them the same way they'd interact with chronological children. For my child parts, being expected to be an adult can be frightening and overwhelming, but it's something they have to do way too often. Only a few people know about my DID, so if someone knows a child part is present, it's because I/we trust them greatly, so it would feel safe for them to respond to the child part as they would any other child.

But that's just me/us, and your roommate may feel differently. Talking about it is always a good idea. She may not have definite answers for you yet, but just having the conversation could be useful for both of you.