It depends how much experience you have as to how much you charge - also what you're offering. For example - when I was fourteen, I babysat for £3/ph, and did some stuff for free to get experience/references. I didn't look after very young children, as it's quite difficult.
You get a range of experiences. I've had kids wake up at sixteen months with teething problems and had trouble getting them to sleep. Done all night sitting, and looking after kids who were sick. I think max no of kids I've sat for was five, ranging from eleven to twelve months old. I charge about £5/ph, but then - I cook, clean and do general au-pair/mother's help stuff as well.
If you're willing to cook, say so. If you're willing to do day-time care, ironing, vacuum, say so. Don't put your hourly rate on the ad - negotiate with each client. You charge something for one kid - more for three kids. It's all relative.