Congrats! We had our first child in his life vest at about 4 days old, in the boat at 3 weeks old, on the zip sled (with dad in tow) at 20 months old, and on trainer skies at 3.5 years old. He is 7, and loves the kneeboard and is asking about slalom.
Our second child was on the boat even earlier (amazingly), rode the zip sled with dad at 18 months, and loves the boat, but has been a little more delayed in interested compared to our first (at 4, he now wants to zip sled).
As Grampa, you just provide the opportunity, the equipment, and knowledge. They will tell you when they are ready. However it goes, it will be as much fun as you have ever had on the water, I'd bet.