How much should I be paying for language and Art tuition?

[fuckbucket]

My ddtwins, Year 8, are going to be getting extra help in MFL and Art from a qualified teacher (not teaching at their own school), which I am swapping for walking her dog. What's the going rate per hour? We are outside the M25.
TIA.

have a look at websites like uktutors and see what the rate in your area - I would say at least 20 pound per hour though

Thanks - whole new ball game for me!

What's making it complicated is that it's a swap, and also that it's for two children not one...