Hi Grandma Dona!
A trip to Disney permanently puts you in the "Best Grandma Ever!" book with your grandkids :-)
As a favorite park for children of that age, I would have to place Animal Kingdom second behind Magic Kingdom.
It seems that children up to the age of 8 LOVE animals of all kinds generally, and Animal Kingdom is the place to be to see them.
They have the Affection Section, an area where your grandkids can touch and get close and personal with the animals.
The Discovery Island Trails, where they can see more of the wild (translation: look-but-don't-touch) animals.
DinoLand USA, which feels like being at a carnival.
The Boneyard, where they search for fossils in the biggest sandbox on earth, as well as go through the various slides (easily my son's favorite thing to do, and he was 2 1/2 on our last trip).
And my favorite, the Kilimanjaro Safari, where you travel through the jungles of Africa and get to see animals in their natural habitat. A top, top attraction for guests and children.
And that's not all!
So, I'd take them to Animal Kingdom and watch them smile from ear to ear...
Have fun!