OYBBK, I'm just chipping in to say it definitely sounds the right decision to walk away from it. I've just done a tot-up, and my kids' little Welshie costs me no more than £40 a week to keep. I've included feet, wormers, basic vet's bills, feed, 3rd party insurance and the amount I would pay for DIY livery if I didn't keep him at home, iyswim. My kids can ride him 7 days a week if they want to, go to shows and PC rallies when we want. My horse's sharer's little girl rides him once a week out with her mum, and for that they pay me half of his hoof care (barefoot atm which works out at about £20 every 10 weeks, but will split the cost of shoes if she rides him more often.) The little girl can come down whenever she wants to, too - it is unlikely that an 8yo, 4yo and 22mo are going to wear out a fit healthy little pony in the prime of his life!
It sounds to me like the yo where you go is using a bit of emotional blackmail on you and your daughter - 'you'll be left behind if you don't get a pony, blah, blah.' As for £85 a week - I would be expecting use of the pony whenever you want for that price. £340 per month would pay for bloody good full livery for a small pony round here (and I am in that expensive Dahn Sarf!) with you just having to turn up and have a coffee on the mounting block while someone groomed and tacked up your horse and led it round to you without you ever needing to move a muscle. And £20 for an hour's group lesson on a pony you are already paying for? I pay £15 for an hour and a half flatwork group lesson with a very experienced lady, no more than 5 in the group. For jumping I go to an international showjumper who has represented Britain on the Nations Cup team, and get an hour in a group of 3 for £20. I'm not sure what you mean, either by 2 free hacks. Is this where she will get to go on an accompanied hack on a pony you are already paying for? That doesn't seem like especially good value.
As has been mentioned, there are so many easier ways of doing it. There are always people looking out for part-loaners and sharers, often for a lot less money, and a time commitment of 2-3 times a week. Another very good option is to find a riding school that is affiliated to the Pony Club. My sharer's daughter does this, and she rides there once a week (PC group lesson for £12) and then goes to rallies and camp in the school holidays. She is 8, and I would say her riding is not suffering through only going once a week! She rides here.