The last time I had cause to worry about such a thing I was 15 myself, and the going rate was £1 per child per hour! I suspect it has increased somewhat since then.
I have 2 children, 2 1/2 and 5. I'll have them all ready for bed but she will need to read a story and get them into bed (they are very amenable so I don't anticipate any problems) then hopefully she should have a peaceful evening. She's 18.
I was thinking £5/hour, does that sound about right?