You offered to look after them and now you want money?
Did I read the same posts from the OP as some people? She offered because the ILs needed childcare not purely because she wanted them to visit so all this "you offer = you pay" doesn't really apply.
It's not like the OP rang up her BiL and SiL and said "Hey, are you doing anything because if not we'd love to have your kids for Easter. Oh by the way you need to send them with £200 each to cover their costs because we intend to do lots of expensive activities with them."
The OP's ILs were both having child care issues for the Easter holidays, the OP didn't have plans so offered to help out the ILs by taking the kids. Under those circumstances I don't understand why it is a bad thing to ask for their ILs to provide some money to cover entrance fees/food costs, especially if both BiL and SiL are aware the OP will be looking after children from both of them and so they will have an extra 5 children, not just the 2/3 of their own.
From going to the cinema to lunch at McDonalds it is going to be an average of £25 extra per activity (£5 per extra child) ... so even if they only go out once every other day that's an extra £75 just to cover their nieces/nephews for 3 activities and that's before the OP's own family's costs and the added cost of basic groceries/extra transport costs are factored in.
OP, I would message BiL and SiL and just say something like:
"Hey BiL/SiL!
DH and I have started drawing up plans for the half term break and we're looking forward to visit. Obviously squeezing an extra 5 kids in the house means it's going to be a week of chaos so our plan is to get out and do as much as possible so hopefully the weather will be nice!
We've looked at what's on during the holidays and were thinking of doing X/Y/Z with the kids. Would these be the kind of things you think your kids would enjoy doing? If so, would you be OK with chipping in to cover some of he entrance fees? As much as we'd love to treat the kids, having all 5 nieces and nephews at once is going to stretch the finances that particular week. If that's not possible or it's not something your kids would be in to then that's fine, just let us know so we can look at doing something else instead.
Talk to you soon! upsideup xxx