For on road it is worth paying the extra for Tamiya TT01s and adding a big foam bumper sticking out right across the front. We run demo cars and the Tamiyas take a hell of a beating, with spares easy to get hold of when they do break and quick and easy to fix. Just make sure you replace the kit pinions with steel ones and the gearboxes last forever. Even more indestructible as we run them on 4 cell nimh packs with cheap 13T EZRun motors. If using the stock 6 cell electrics or lipo then going to cheap 55 turn motors will slow them down again. Both will also give you much greater run times from the battery packs.
For tough buggies, again I would look at Tamiya. The Grasshopper, Hornet and newer Mad Fighter are virtually indestructible with again easy access to spare parts from anywhere in the world. The Sand Viper/Desert Gator/Neo Falcon are more complex and more expensive but they are designed for bashing so pretty tough and again easy to fix and easy to buy spares.
It's more important that you have any easy and quick supply of spares than having something pretty indestructible but can't be quickly repaired when it does break. Not much point in buying a Maverick if you have to buy all the spares from the UK, and good luck finding spares for the Chinese makes anywhere in a year or two.
Supastox will also work great, just swap out the kit motors for Axial or HPI 55 turn motors and change gearing and run them on 4 cell. Make up bigger foam bumpers that stick out the front and cut the body away so the body rests on top and you will hardly ever have to replace shells.
Adjusting speed by using EPA is never going to work, you have to have something physical to prevent it. The ones we use have stick radios with a self tapping screw added just above the stick so the stick hits it and it stops you pushing the throttle forwards. Give the car to someone with some driving experience and you can wind the screw in to give them more speed and the only way a kid can adjust it is if they have a screwdriver on them. Steerwheels are a bit more tricky to modify, and if you put the adjustment stops inside the case then they can't be changed. You can also replace the EPA adjustment potentiometers with smaller PCB versions loose inside the case, so you have to dismantle the case to be able to adjust them.
Members of my club run r/c car tracks at most of the classic car shows in the area, for grass HPI Blitz short course trucks are used, again with slower motors and lower voltage batteries to slow them down (4.8v and 540 motor or 7.2v with 55t motor) and again with big home made foam and plastic bumpers added poking out the front to reduce damage. We have very few breakages. A fun fair that saw the r/c cars thought they would do the same and bought the same trucks. They ran the kit standard cars and soon found they went too fast for the kids, and then didn't know how to fix them when they broke so gave up and we now have them.
I would also suggest looking at the track design. Solid barriers are great for experienced racers but the kids aren't too bothered about the track moving. The photo above shows many spots, such as the ends where there are gaps in the barriers, that are almost designed to take the corners off the cars. Adding full size car tyres at the ends will give a softer barrier so the car won't get damaged on impact. A ring of tyres around the pillars will also stop any damage.