We're looking at one of those mini Jeeps which are classified by the sellers as a go kart. My boys are 9 and 6 and we have 4 acres and a gated neighborhood for them to terrorize.
The one at tractor supply is by Massimo, and there's a Coolster as well, both 125cc 4 strokes and pretty similar. My wife leans towards the Coolster because it has a roll bar and shoulder belts. I like it because it has a chain guard which will help when they drive it in the dirt and rocks on our land.