All that free childcare and they are not even giving you the cost of feeding them? With respect, your children are leeches!
I used to be a childminder. When it started to become too much for me I retired from it. When my daughter and SIL decided that they wanted to start a family I made it clear that I would not consider being a full-time carer. I would gladly do the odd week or two, cover for sickness, and babysit now and then, but that was IT! They were perfectly happy with that, and the children went to nursery.
If you can't afford to pay for childcare, then tough - don't have children! I think that most of we GP got little or no help from our own parents. I was a stay-at-home mum, and child-minded to provide some extra income. Money was tight, but we managed by careful budgeting.
Tell these selfish people that you can no longer cope with the amount of childcare you have been doing. If it hits them financially, well, that is their problem. They have taken advantage of you for far too long.