M.R.
The gracious thing to do would be to offer payment for them to babysit. In my opinion, it really needs to be their choice as to whether or not you return the favor. And...it should be cash. If you offer payment first, it is likely they might decline it due to the situation and, at which point you can give the "Oh my goodness, thank you! That is so kind of you - I definitely owe you one big time!"
Yes, it would be an expensive night but if finances are that tight you can always have a few alternatives:
-Don't go to the wedding and just send the gift
-Go to the wedding ceremony only (to reduce babysitting costs)
-Go to the reception only (to reduce babysitting costs)
-Go in with another family member who is also attending the wedding, have the same babysitter for all the kids and split the cost