No, I wouldn't on a regular basis twice a week. I absolutely adore them, and will happily fly across Europe to visit them and to look after them in an emergency or special occasion - but my daughters totally agree that is is not our job to bring up their children. We are part of the process, and love them to distraction - but no, we won't bring them up, and our daughters would never ask us to. The role of grand-parents is to support, lots and often, but not on a regular basis. If grand-parents are young and active, they have their own life to lead after all the hard work - and if not, they shouldn't as it is too tiring.
Sounds selfish? I don't think so, and so do our daughters. As said, I'd jump on the next plane in the middle of the night if needed in an emergency- or would look after them for a whole week or more, or week-ends from time to time. MHO.