not sure how you feel, when my boys were 6 and 9 we went to gas powered 4 wheelers...they had a kill switch on them, for added safety, and gave the boys the speed they were craving. i was aprehensive, my hubby talked me into it, austin (11) can no longer manage the pedal on it, but it was the best two summers of his life according to him. now we are looking to modify somehow so he can ride again. just for the record, he did crash a few times, but because we had a speed govener on it, it was never going so fast he got hurt. i know this stuff is contriversial, but his smile was so worth it because he could keep up with siblings and friends.
We bought Daniel a gas powered quad when he was 6 yrs old. Always very cautious about everything, he drove it very slow around the yard for quite a while until he got comfortable on it. He drives it really well now and while I had some apprehensions at first, they quickly faded. He has never been reckless on it. I think it was one of the best things we ever bought him. We have a lot of hilly yard (20 acres) and a scooter or chair would never be able to traverse it.
We actually live in town but my entire family lives in the country and my kids spend alot of time there. My son will be nine next week and has pretty much outgrown all of his ride-ons as well. We got a Chuck Wagon last month, a 2WD model. Kind of a cross between a Polaris type vehicle and a golf cart. It's gas powered but has an electric start, though I believe they may make an all electric model. It does have seatbelts but my son is too short to be able to use them and drive so we're having the seat or floor modified, not sure which. And I believe they're putting a governor on it for the speed as well. My family actually bought it and is having it modified, so I'm not exactly sure on the specifics. My son absolutely loves it. The only negative things I can think of is the gas pedal can be hard to push, which isn't such a bad thing with controlling speed, and it was quite stiff when putting it in gear but that seems to have gotten better. All my son wants to do is got through mud / puddles and it's been great for that. lol
How about a mobility scooter. Pride Mobility has many different ones. Check out www.scooterville.com for scooters. My son has had one for the past years. We just bought a Pride GoGo Elite Traveller for basic use and also a Pride Revo which he can use on grass and dirt. We call it his ATS vs. ATV. He loves his and has it souped up with fire decals and decals of his favorite baseball team -- the NY Yankees. He thinks it is cool as do all the kids. hope this helps