Elders can already raise or help raise their grand kids. There are also tons of things for elders to do, since they can do almost anything adults can do. They can have a job, socialize, enjoy most hobbies (other than fitness). I often play three-generation families and, when I don't need their help with the grand kids, I'll just leave the elders to their own devices to enjoy the hobbies that they've picked up over the years. Now and then I'll get a notification like "Grandma painted a masterpiece!" or "Grandpa hacked into the Landgraab Industries mainframe!"
I do like the idea of elder-specific mini-aspirations. And I fully agree that we could use more activities for the little ones.