I love yh but I am a member of the SYH because it is cheaper and you can still use YHA places.
If you want your family of say 5 to sleep together in a dorm it is well worth paying for the extra bed in a 6 beded dorm because you never know, they may put some on in that bed. (It happened to us in Bratislava once!)
Even though my boys are older Eldest is 14, I prefer it if we all sleep in the same room.
You don't have to take your own sleeping bags any more they provide the bedding.
The kids favourite YH is at Arnside. I have stopped at Keswick several times while I have been away working and that is a good base.
There are so many different types of hostel from city centre ones such as Manchester, Liverpool and Edinburgh, They are all good, to remote isolated ones such as Black Sail which I wouldn't recommend if you have very small children for the following reasons:
'Due to poor reception YHA Black Sail is a TV free zone. There are no electric sockets or electrical items in the kitchen and mobile phone coverage is very limited. No credit card facilities at the property. Meals should be booked at least two weeks in advance but prior notice would be appreciated - meals are from a 3-course, one-pot menu. Special diets can be catered for if informed in advance. Drinks are available during reception opening times and only alcohol purchased from YHA may be consumed on the premises. Bedrooms, shower and toilet are all accessible from outside - bring a torch and light outdoor footwear.'
I love it there though it is really great.
Have some good adventures!