Varies by where you live, whether these are relatives or friends or neighbors or just a job. If I was the parent, I would expect to pay less on a weekly basis than I'd pay for an evening (which is now about $6 per hour in my area.)
In California, you are supposed to have a license to care for more than two unrelated-to-you children in your home. Be sure you check your homeowner's (and auto, if driving the kids) liability insurance to see that you are covered in case of injury.