I'm more than happy to take care of my GD when needed, but I wouldn't like it to be on a regular schedule, unless it was absolutely necessary. I like my own space, and my own free time. I do find that keeping young children occupied for a whole day is extremely exhausting. I simply don't have the energy in my 60s that I had in my 30s! My GDs are wanting to be doing something every minute of the day. They want to make cakes, then before that's finished they want the paints out, then the dolls house, then the kitchen, then the lego. And, of course, Nannie has to play with them too!
I literally don't have a moment to sit down with a coffee for a 5 mins break, and usually don't have any lunch either!
I think in the OPs situation, I'd be inclined to say that I'm happy to take GD1 for 2 or 3 hours, or for a morning or afternoon, just to continue with the routine, and for me still to have that special time with GD. Also, considering that your DH has recently had surgery, just say that a full day is a bit much for you to handle right now, as he needs his rest and he's not currently able to help out with GD. I think it unnecessary to expect you to have her for the full day whilst DiL is on maternity leave. Just put it to them nicely, and I'm sure they'll understand.