Did She Just Tip Me?

I help out at Ms. R's school nearly every day in one way or another. Copying, laminating, coloring, grading, sorting ... you name it, I've probably done it. Today while Ms. R's class was off at "specials" I went to hand out some paper work and had to swim through several disastrous desks to find their homework binders. I'm proactive, so what do you think I did? I decided to clean out ALL of the kids desks. Ms. R's teacher was laughing at me. Yeah, I have issues, what's it to ya?
When the kids got back, she had them look in their desks, notice how nice they looked and reminded them to say thank you.
One of the little girls came up to me about two minutes later and handed me $1. A bit confused I asked her what she wanted me to do with it.
"It's for cleaning my desk."
"That's alright, Bree, I cleaned out everyones desk. You don't have to pay me for it."
"You did a good job, you can keep the dollar."
"Why don't you keep the dollar and as a way of paying me back, keep your desk clean?"
"Nah, you keep the dollar. I'm not going to keep it clean."
"Fine. Just put the dollar down and if you change your mind you can come back and get it from me."

I have no idea why getting paid by a 7 year old for cleaning her desk makes me feel cheap.