I think this is seriously expensive - it was a free flow art and crafts session (extra £2 for face painting). Granted they need to buy materials but it was in a park and there must've been about 35 kids. Strangely if it had been £6 for an hour I wouldn't have minded as much. I asked other mums if they thought it was poor value for money but they said no they didn't mind because they didn't have to clean up - 
BTW we are in London but it was in zone 3 so not central.