I nanny for this family and the kids want to go to the movies. We asked their parents and they've said yes.?

They're totally okay with me taking them to the movies, but they haven't mentioned paying for the ticket. This is will cut into my Friday night and I'm okay with that if they're going to be paying for the movie ticket(and theirs). Should I politely say that I can do this only if they're agreeing to also pay for my movie ticket? (Afterall, I am a poor college student.)