And given your advice about the taxis, and the fact that land-based sightseeing Grand Canyon with a 4 year old might be challenging anyways, so I'm considering an option when we overfly it, land on KGCN to have some rest, and then head directly to Flagstaff.
I'd try and see it from the rim, standing there quietly on the edge is something you'll remember. Its a long way to the bottom or the other side
As already noted, the hotels in Tusayan are not far from the airport, although too far for a 4 year old to walk. The helicopter ride FBO is typically buzzing with people and I've got to imagine that they could sort out a taxi if nobody else was around. It also wouldn't surprise me if somebody gave you a ride in their car. Once you're at one of the hotels in town you could get a Jeep tour if that appeals. The leave right from the hotel area, and although not cheap might be good with a youngster.
Foreflight will make life much, much easier.