Hey, as others mentioned, it's hard to judge without knowing the situation.
You say "my children" how many of yours is she looking after, and for how many days a week? I ask because if she has two for five days a week, I would charge you £360, which potentially makes £270 look very cheap.
I have a rule in my policies, not contract, that I will do activites with my children every day, if I am charged more than £4p/child I will invoice parent. This is mostly because I have four or five children a day, and if I had to pay for all of them two/three times a week, that could range from £32-48 (big expense every week). We also in the summer do bigger trips, which include getting on train to central London, going to museums, aquarium, palace, and going to the seaside etc so those I charge for. Parents are happy to pay, because they know children are going out and experiencing new things.
Is it payment your not happy about, or the fact that she goes to the club? HTH