Just to add to Steve's post, if
you do try to terminate the contract now you would be liable to pay the remaining months x current tariff in one lump sum.
BTW, I'm still curious about the calls part to get that expensive. As you changed tariffs, what was the billing date and pro-rata payments (will be on the bill) stated? There may be some problem over that which would have resulted in charges, though it should be small.
Leading onto my next query as to what the allowance was at that point in time as using normal calls would have used up the allowance:
http://shop.o2.co.uk/tariffSmallPrint#incMCPM before charges being applied.
Also - and as a point to remember - has one of the numbers that was called, migrated to another network from o2 without the granddaughter knowing? If so then the o2-o2 calls isn't going to help in the future!