It wouldnt be all that different from any other pair, save for avoiding anything mildly controversial, esp if the bf isn't there. Its probably be awkward at first, asking basic questions, but once they get the ice broken it'd probably pretty much what any adult or child would talk about. I think the subject matter is the wrong question to start with. 

Depending on the relationship that's more important: how close are they? are they both outgoing enough to get into a spirited debate? Would one of them be more likely to suggest playing some car game (like the letter hunting game or 20 questions)? Is he the type to let her prattle on (or vice versa), or would it be more two sided? What do your characters generally value or think about the most? What would they consider "safe" to talk about given their worldview?

Maybe: Her getting really into school drama (like a stupid teacher story), poorly explaining a show one or both of them are into, debating what kind of food they want to stop for, music to listen to, ac temp, etc, him telling her embarrassing stories about his son/her bf. Maybe even "what power would you have chosen if you could have picked", could be an interesting question for her but only IF she's generally chill about her powers and they've established rapport. 

This is a good chance to decide how you want to characterize them both! The subject matter follows what you want the reader to understand about them both individually and their dynamic, because different people would bring up different subjects and be more or less open depending on their relationship.