I have a plus gym membership which is the 2nd cheapest (cheapest is club which is off peak hours only) and I get a discount through work, pay about £90 but I think without discount would be over £100. Children under 2 is £2 per month and then children 3-16 years starts at £36 (I only enquired about prices a few days ago) I havent tried any of the children's facilities so can't give opinion on that however I have taken little one swimming lots and gone to the lessons (this is extra) you could be looking at paying around £280ish a month for all of you at an estimate
My mum's a member. Prices have just gone up loads for current members. She's stopped one child's tennis lessons. My children are on her membership. She has the membership where they get towels for swimming. My kids both do the clubs and love them. Membership seems to really vary from person to person. My mum said recently they're oversubscribed and it's rated the best DL club in the country. She also says it's gone from being mostly older members to all families.