My parents are 300 miles away and my PIL 5 minutes down the road.
My sister lives round the corner from our parents and they do 3 days of childcare a week on top of weekend babysitting and stay overs so my sister and her partner can go out.
My PIL does very occassional weekend baby sitting and currently has DD one afternoon a week so I can have a bit of time for myself (am currently on maternity leave). However my FIL is ill and quite frail so I don't feel it's appropriate for them to do any more childcare, although I know MIL desperately wants to, especially when I return to work next month.
We are in London so nursery is very expensive at £55 a day and £35 per 1/2 day for under 3s - you book by the sessions you need but it's loads cheaper to buy a full time place.
As others have said, I wouldn't want to be given any money from my parents to make it fair. Instead it's just good to know that both sets of parents can help us out if needs be in an emergency or give us an evening out by babysitting.
What rankles me is when my parents can't do stuff they want to do, like go on holiday or come to visit us for a weekend due to my sister's childcare/babysitting expectations and her inflexibility or willingness to take days off work/pay for nursery on what she considers to be non nursery days, but that's a whole other thread...
You sound like a wonderful mum to be trying to be so fair to your 2 daughters, but really all that matters is that they know you are there for them to help out when they need it and that you want to be involved with your grandchildren.