Hi
We've stayed at Youth Hostels a few times - me and 3 kids (aged 2-10).
If you get as far as the peak district then.....
Hartington Hall - awesome - an old manor house, really nicely done up. They have a few animals (goats and chickens I think) in a courtyard which the kids loved.
Ravenstor - old rambling place, big grounds, great walks from the door or nearby.
If you like cycling, there are several excellent trails on old disused railways lines. You can hire bikes on most of them (one v. near to ravenstor).
Cholderton - close to Stonehenge. On a rare breeds petting farm with softplay etc. Quite modern inside. When we went, you could spend as much time as you liked on the farm included in the cost.
There are some great looking ones in Wales that I had been eyeing up but as we have recently got a dog that isn't going to work!
Do you collect Tesco Clubcard vouchers? You can use them to pay for your trip.
Meals - I tend to take frozen bolognaise etc from home and self cater for a couple of nights and then use the restaurant for 1 night. They usually offer a free kids meal with every adults one (older child might prefer an adult meal.....).
I think they are brilliant places. Usually a few other kids around for instant playmates, and plenty of adults for a chat. I've never felt out of place being on my own.
Have fun!!