What would you think was the rationale behind this? tipping

I would assume they know the kid on one side personally. It isn't just a kid drying cars but a nephew, neighbor, best friend's kid... something along those lines.

They assumed it would be pooled then halved but wanted to put something in each kid's hand - stretching, I know

THe $5 side looked dryer?

I think that is bad. If they are drying the same car, you should tip the same.
Very odd that someone would do that.

That's weird. Of course, I think a $7 tip for what was probably a $5 car wash is weird as well.

I can't imagine asking for change on a tip.

I would tip both workers the same or not at all.

I think you are overthinking this. It was one tip from one person, when your daughter was drying cars. How good/bad could she possibly be at drying cars?  

Here's my take on it -- people are just odd. I see things each and every day that makes me think that quite often. I don't mean it as anything more than an observation and a way to amuse myself with people watching. Here's a prime example. Last week on the train, there was a guy sitting there with both a bucket style hat and a bicycle helmet on. He's on a train and obviously doesn't need either one. Oh and there's a wide open shelf to store personal stuff so it wasn't a lack of room to put at least the helmet somewhere. No harm, no foul, not my business and it doesn't matter to me the whys of his head gear but people are just odd (IMHO).

I would let it go. There are a bunch of crazy and not so crazy reasons why the person may have done it, and you'll likely never know which, so just go with the person being a little odd and don't make it be about DD at all.

It was just drying a car.