I think it is totally unreasonable to expect grandparents to do as much. I adore my DGSs (now nearly 4 and 2 and a bit) but the longest we have looked after them was for a long weekend when DD and SIL went to New York (!) and the boys were 2 1/2 and 1, and I got help from another DD and her BF, partly as DH is not a well man , can't lift, walks with sticks and between you and me, is not and never has been a lot of use with tinies, but that is not the point. I have also had them here for 4 days (DD and SIL in Florence - they know how to live) and my Sis-in law came to stay to be my extra pair of hands. When DD (obviously fishing) once asked me how old she and her younger sister were the first time they stayed with Granny and Grandpa I could reply with total honesty - 6 and 4 - and being girls they were easier to look after than 2 little boys who are apparently fitted with Duracell batteries like the bunny!
If you didn't mean overnight and they are at nursery/preschool then it might be different, but it is so hard when one is asked to do more than you are comfortable with, as a "No" sounds as if you don't want to, not that you can't quite manage.