Is this appropriate?

My NF needs care over 50 hours per week. They know I don’t need or want that many hours, but love them and will do it. They don’t have any other nanny/babysitters, just friends for occasional help. 2 kids under age 2. I want to know if it’s appropriate if I ask to get a family gym membership so I can use the gym childcare during my shift? I’d happily pay for it, and would put the kids first always (not messing with their nap schedule, stay home if sick, etc), but 11 hour days with no break can be exhausting. (I don’t necessarily think they’ll say no, I just don’t want to be wildly offensive or something, and would love to be prepared for any questions) Advice please and thank you!!