age gaps

Swimming, if the eldest can swim and maybe invite a friend along for him so you can concentrate on your GD. At 12 they often want to be with their friends.

I have 6 grand children, with ages up to 12. I often look for scientific experiments (loads on line), using everyday items such as balloons, drinking straws,string and Velcro. The most popular are the messy ones, best performed outside. They often involve vinegar and bicarb.with which you can make volcanos and space rockets with dramatic results. Adult supervision is, of course, essential!
It is great being a grandmother, as you can devote your whole attention to them and clear up when they have gone.
Good luck for the school holidays, hope this helps.