I like this question - and wish there was an option!
I play GTA V with my son (he's 9).
Before anyone moans - the way I play it is I created a 'deathmatch' in the desert with no weapons, but with a load of ramps and a wide variety of cars and planes. I then go into the test mode, and that enables you to test, obviously, the environment, but without being online with other playes. I also turn off all pedestrians. The reason why I create a deathmatch is because that seems to be the mode that you have the most control over - he doesn't do any fighting when he plays.
So in this mode (that we only play together - he doesn't get to play it without me with him), he gets to fly the planes and helicopters all over the map - which he loves, and also try all the different cars and do jumps but also just drive around the city, along the beach, off the mountains...
The test mode can be played for a max of an hour, so it actually works well.