Paying for someone else's kid at festival: what would you do?

You're at a fall fest with two friends and their kid, as you getting ready to leave [in your own car, with which you are going directly home] they friends need a bathroom break and ask if you will walk thier excited kid around the booths real quick. You say yes. Mother tells child "Go see if you want to do one more thing with auntie [you]." This was assumedly said to calm the anxiety of the kid.

You walk around the booths doing assumedly what you think you're supposed to based on what was said to you and the kid. Which was point out *one more thing* kid wants to do before we leave. Child wants to get face painted. You say ok and about that time parents walk up and child has face painted.

You're talking to your friends and their kid is done getting his face done. Face painting lady wants $36 dollars and parents look at you and continue talking. Face painting woman looks at all of you, obviously thinking why is she being ignored. You think maybe mom and dad didn't hear them and politely say "I think she's trying to talk to you", to which you get a weird look, then a look like you just shot their dog and dad pays the lady.

Apparently they assumed you would pay for it, which was texted to you later while you were driving home. Now they can't get thier older kid a gift or something.

Would have have assumed you were paying? Keep in mind I am still in college and 10 years younger than them. I even told them I may not be able to join them do to a lack of gas earlier.