Oh man, do I ever hear you on the idea of having the parents replace whatever gets ruined. My carpet looks absolutely disgusting since I started caring for my little "puke baby" about 5 months ago.....however, as much as I would love to tell the parents they must pay for a carpet cleaning or daydream of handing them a bill for it.....it is the down fall of the job. Babies puke, kids have accidents..that's part of what we signed up for when starting this job. I would take out the "replacement" part of your contract.....as much as that stuff can anger us, it does come with the territory.
I do however completely agree with the comment made about, sending the child home if they don't come dressed proper. That makes perfect sense...these parents are obviously NOT LISTENING to you, so make them! Good luck.