I think that for their ages that Silver Springs is great. Much better that Disney (for their age) much better than Homosassa Springs ,my opinion, and closer. Have been to all. Be sure and go up Baseline Road off of 441/27 and it will take you right into Silver Springs. Fastest way from The Villages.
The first visit for kids that age should be the Wilderness Trail and ride the Jeeps in the Ocala National Forest, go past the boat rides and directly to the Wildnress Trai,l then go to the zoo area. Then if the kids have any energy left go to the boat rides. Also walk them to see the gators lots of them. This area is to the right as you enter. But go to the Safari ride for thir ages and yours you will enjoy. This is where they filmed several Tarzan movies.
After you are inside you can decide if you enjoy it and you can purchase a year pass for very little more than you paid to get in. If you have other grandkids that come down you can take them and not pay your own way in. Also will get you into the great shows that they have there. (Check out website) Great Deal! Or if your Grandkids come back before a year is up they will want to go back.
Website below. A must for grandkids that are below the age to appreciate the Orlando attractions. Go in the AM and by noon they will be ready for a nap with you. Again just my opinion.
http://www.silversprings.com/rides.html