I had a two hour trial and was paid in cash that day, don't refuse payment, this is your time!

I'm a nanny who babysits quite a bit on the side. I've found a family lookin for regular care one night a week, and after speaking to my references they have asked me to come do a (2 hour max) trial run tomorrow morning, to see how the children react to me (they are young, I will be the first non-family caregiver). In the past, parents have always tried to pay me for these trial runs an I have always refused, but they have also only been 30 mins or so of me hanging out with the kids with a parent in the other room. I don't know if I should still refuse payment for 1-2 hours of work.