Being the capitalist pig that I am, I am having a hard time finding fault with this, beyond the potential embarrassment from getting caught and called out.
The 'rented' kid gets room, board, a free trip to Disney, and 130 bucks an hour. I'm even having a hard time labeling this as immoral, as the kid is genuinely handicapped, and if the kid went to Disney with his/her actual family, the kid would be treated exactly the same way.
This sounds like a pretty lucrative summer job to me.