I would say it depends upon their age and experience, what you expect them to do (simply play with the kids for a couple of hours while you run errands, or watch, feed, bathe, and put them to bed until you come home after midnight), and how many kids they're going to look after.
I would expect to pay a 17 year old more than a 13 year old.
In the US right now, my brother pays his baby sitters (usually college-aged kids) about $10/hour.
For a younger teen, I think I would probably pay around QR 25/hr.