Wow, you are very tight fisted. Expecting payment for looking after your own DNs? Bizarre
I never understand this way of thinking, it always seems to go in the favour of the CF's, if the OP is tight fisted then what are the relatives for not offering?? Surely it goes both ways? You want to avoid a family 'argument' but they don't seem to mind risking this, even though its THEIR children ... thats whats bizarre!
At first glance I thought you were U, but reading it all properly I can see how you got into this OP, you offered to care for them not even dreaming that the parents wouldn't automatically expect to pay for their childrens sustenance and entertainment - I completely see how this happened.
I think as it isn't 5 from one family, ie only a 2 and a 3 that they're not realising what a big deal it is. I definitely think you should broach the subject with them, after all they'd have to pay out if they had their own children wouldn't they?
If you don't get any money to cover basic costs then they not only get a child-free week but they even save themselves money into the bargain. You also have double the petrol costs as well and this could end up costing a fortune, as you say £500 for entrance for one day, even before food and petrol! You'd be paying for the privilege of having a hell of a week with 9 children. And you will be substantially out of pocket.
As someone else suggested, I'd just email them both in the same email and say you were thinking of doing x and y and asking if they were ok with the cost of it, as if you haven't even thought that they won't pay anything at all if you see what I mean. So, for example, hello everyone, re the holiday, we were thinking of going to x which is £20 per child and y which is £25 per child - is everyone ok with this? Then you could add something like 'we can take a packed lunch to keep costs down unless you'd like them to eat out. You could even say "We don't have to go out, as I appreciate that this could be more than you were expecting ...." which sort of implies that they obviously would have been expecting to pay something to cover their food at least and it brings the issue of money up.
That makes it clear that you're expecting a contribution anyway, even before trips etc.
But if they pay nothing at all, then its them, very definitely them who are the CF's! You really do need to speak up OP.