Although you were nice and offered to have the dc it does sound like it was a bit of a set up by your BIL and SIL - telling you all about their problems. Not sure if you had already decided to have an easter at home or have done so because you are having their dc?
Did your dh already know they were having problems and get you involved in the discussion so knowing you were nice you would offer to help out?
What would they have done if you'd said good luck, we'll be thinking of you while we're on holiday?
You didn't wake up one day and suddenly think, out of the blue, I know, let's invite 5 dn to stay with us for almost a week over easter, they could easily have a lovely time with their parents, but no, we'll ask them to come and stay and be entertainment for our dc... If you had, then you probably would have been unreasonable to ask for costs or at least to ask for lots.
On the other hand, you were told about sibs in law having problems with childcare and offered to help out - and while your reply might not have been spur of the moment, I bet you didn't have much time to do lots of detailed thinking and planning and working out costs. In which case, it's entirely reasonable to say 'Having just had half term and seen the extra costs compared to going to school, and knowing what we had already got planned and were hoping to do for easter holidays, and now we've had a chance to think about doing you a favour over easter the implications of having so many kids at once, it will be lovely but fab but expensive, so would be really grateful if you could contribute £xx. Look on the bright side, if you hadn't asked us or if we hadn't been able to help, you would be looking at having to pay £££££ for holiday childcare, so all things considered I think it's a pretty good deal!
If they are expecting you to pay too or are trying to justify it to themselves as you wanting to do it then they are definitely CF and have reeled you in good and proper!