This is not advice, but simply a tale of events that may, or may not have happened.
There once was a guy from Nantucket.......oops wrong fable.
Haahahah....anyhow, if your grandson is capable, maybe a "booster seat" can be fabbed up so that he can sort of "ride the front" of the seat and still reach the pedals while still utilizing the seat belt? (oh you might have to defeat the "seat switch' too)
I started my grandson on the backhoe and just recently he started driving the tractor. Our land is pretty flat and he just likes tearing up/driving around the back yard, and digging holes with the backhoe so he can fill them in.
Edit: For the record I drive the tractor out of the barn, and around the cars, to avoid a "side swiping" event. Then he gets on it, when its "out in the open".
He has a blast.
Sorry I have no idea about where to find an electric one though.