I really don't think grand-parents should be 'expected' to take care of grand-children on a daily basis. If they want to, fine - but this should be respected and rewarded, if not by a salary as such, by treats.
We have several friends both here and in the UK who are totally trapped by child-care. One couple bought a motorhome when they retired to explore Europe which they never had a chance to do. She then had 3 years of battling against breast cancer and finally saw the light at the end of the tunnel. One son then asked them to help look after the little ones 2 1/2 days per week. Then the other returned from abroad and told parents, if you look after his, it's only fair that you look after ours the other 2 1/2 days, and then felt they couldn't say no. Similar story for several friends in UK.
Recently they gave them 2 months notice that they were going away for 2 weeks with said motorhome, and both sons went crazy saying 'you can't go, we do not have an alternative for the kids and don't trust anybody else'. They finally put their foot down and said enough is enough- they'd continue to help, but they'd like a bit more appreciation, and that if that alternatives had to be found for when they want to go away for a few days or couple of weeks. BRAVO. For them it is NOT about the money- but about time and freedom.