My children have always been looked after at least 1 day a week by their grandparents, and now they are at school, it has risen to 2 days (grandparents do school pickup, and we pick up kids from grandparents a couple of hours later.
We all LOVE this.
When I went back to work I felt incredibly sad at leaving my baby with a stranger (perfectly nice childminder). On the days he was with his grandparents, I felt totally different, as he was with family, people who actually genuinely love him, and are (in the case of my parents), an extension of myself. Short of staying at home myself, this absolutely saved my sanity, and stopped my heart breaking.
My kids have an incredible bond with their grandparents, and their home is my kids second home. They have toys, books there etc - and the children love going, as it's just a bit of a break in their surroundings, but with all the comfort of home.
And they love their grandparents, and those extra relationships have added a whole dimension to their lives (eg they play cricket in the garden with grandpa, and have now started stamp collecting following conversations with him, etc).
For us it was also important that the grandparents did not do ALL the childcare, but only a couple of days - so they still had plenty of freedom to do their own stuff, and the grandchildren were a pleasure, not a chore.
I know another mum at school had an arrangement where her inlaws would pick the kids up from school on the monday, keep them overnight and drop them off at school the next day, and she would pick them up from school - giving her a good chunk of time to get the work she needed done without the day being fragmented by pickups and all the nightly duties (she was studying for a professional exam). I wouldn't have wanted that myself, but again the kids and the grandparents have an amazing bond.
We are often pushed by work to relocate to another country (expat roles), and in fact the main reason we turn down these offers is because we don't want to break this bond with the grandparents which is so precious, and finite.
This is very much a cultural thing too. My husband is French and in France the grandparents often take on the children over the holidays, bringing together sets of cousins. People will literally send their kids to their parents for 2 weeks, and you see grandparents on the beach with about 9 kid under 10 :)
I think you are doing ABSOLUTELY the right thing.