Thinking about it a bit more.
So they have 4 gcs, and they think that's it. With 4 gcs we can afford to tive each gc money every month, so they set it up. They're not expecting any more gcs; maybe dh's ex has said she won't be having any more, and bil has said that too.
And then there's the split, and then their son marries again, and lo! more gcs. So they duly give money to their 5th gc, and then their 6th, by which time they're thinking "oh goodness, this is not what we expected and we don't have the dosh, and now they've had a third (our 7th gc) and how many more are they going to have? And for each we're going to have to give more money we can't afford aaaaargh better stop this now." They've done it gracelessly. Maybe they're really panicking though.
If you won't have them at Xmas then you face the danger of confirming that you're only nice for a price.
Again, having a real relationship with gps can be incredibly fulfilling and enhancing. If the only thing is the money, then I'd ignore it and concentrate on building good strong bonds with them for your children's sake.
If this is only one of a huge number of difficulties the ILs carry with them, then your children aren't going to be helped by a few quid here and there anyway.