The core of the idea seems very useful and I want to draw a picture of my two aunts who take care of their grandchildren. Their story may supply an insight from another perspective related to this concept.
Me and my family, we are living in Istanbul/ Turkey, where people work for long hours and waste so much time on traffic. This means most of the parents can't take care of their children by themselves, here. Even though they can afford a babysitter or a helper; most of them prefer their mothers and fathers to raise up their children - also I should note that people I know who are in their 60's-70's are really energetic and they are still passionate about life.
Nesrin is 66 years old and she helps her doughter to raise up her 3 years old son Demir. Last year, Nesrin was able to go to swim and do pilates in addition to carrying her responsibilities; because a helper was supporting her by sharing some of the house works. After helper had leaved and Demir started to go to a baby day care center -just from 08:00 to 12:00, she gave a break to do sports; because somehow the number of her responsibilities increased.
Nermin is 63 years old and she raised up her granddoughter Azra. Nermin was quite busy with Azra when she was a baby. Now, Azra is 8 years old and how they spend their time is quite changed: Nermin goes to sewing and embroidery courses in addition to carrying house works and Azra plays instrument, goes to school. They are still close friends, some nights Azra stays at Nermin and they sleep together :)
So, my aunts are willing to take care of both themselves and their grandchildren; while they are willing to spend time on other things such as doing sport, sewing, meeting with friends etc. On the other hand, with respect to their age, it is hard for them to synchronize all of these activities without a support. To sum up, regarding my aunts' context, this center can involve options such as scheduled courses and activities for healthy grandparents who come with their grandchildren just for three days in a week or a specific period something like that. No need to cook, no need to wash dish, no need to feel responsible etc. - just enjoy themselves while they are sharing environment with their grandchildren.
Apart from these examples; glass separators would be nice rather than opaque walls to turn a flat into a space with many rooms: glass can absorb sound while it keeps everything is visible. Besides, it may turn out to something bad, cause I have no idea about glass surfaced interior's cost or safety.