i don't often put money in the machines, but my kids like to have a play on them. I don't want my kids to think they get a ride every time we go to the shops, and i don't always have the money for it.

In the original scenario, if my kids were already on it, and the next child didn't want to wait for their go to turn the machine on, then i wouldn't take my kids off but we would thank the parent and child. Otherwise, i see it as we are not as entitled to play as the next person with cash. I do get embarrassed though, and would prefer that they just not offer when my kids are already having fun without the movement.

If i am paying, i have no hesitation if another kid is jumping on and would not want $1 from the other parent or a second ride. if i do it, its because i want to.

Another thought, for some people chit-chat or saying thank you is not always easy. There might be embarrassment because they couldn't afford the ride that day, talking to strangers is difficult, or maybe their head is just not in a good place. Take the child's enjoyment as thank you, you might have given the parent 2 minutes of peace that made her day.