U can ask 4 anything
Often grandparents will be happy to babysit their grand kids for free--to some extent, but some people are not so lucky.
Baby sitting service is expensive, because it is a lot of work. So, on/off, here/there babysitting is often offered as a freebee, but grandparents don't have to do this--not a law, just a common thing. Many don't ask for it, but they have the right to ask for $$$. Good luck.