Trashy Pickups - Kids don't count as people on an UberX

I saw this circulating. Typical story as an Uber driver, of course.   

However, there are two elements to do this and I find it a bit interesting. One is the trashy people. Gosh, I hate dealing with people who have no manners, lack commonsense, and are just cheap.

The other is people believing that children don't count and they don't have to pay. Is this a thing? Is this a common misunderstanding from passengers?