I can't answer the questions you wrote, but need to make a comment as the father of two avid cyclists whom I got started at very early ages. (Now they are 19 and 22 years old and still neither one has bothered to get a driver's license.) They were heavily into cycling before the age you cite. The summer the younger one was going into 2nd grade, we regularly did 50-mile rides, and he rode 170 miles in a 3-day stretch that summer, in spite of still being on a single-speed with fat, knobby 20" tires.
It was not until they were about 11 however that something seemed to click and there was an almost sudden ability to make good decisions in traffic. Until then, I occasionally took them on rides that required a little more traffic skill at some point in the ride, thinking they were ready for it, and then some scary situation would arise, telling me they needed more time before we try that again. Then for the next few months, I would keep them on paved trails and roads that had little traffic, but continued to coach them on safety issues. They knew they were way ahead of their peers in cycling (and were proud of it), but they also knew they still needed the coaching, and did not resent it at all. Fortunately there were no actual accidents, but the need for more brain-development time really showed. After they were able to be on the streets and roads regularly, I still coached them a lot, and they did not go out on major rides by themselves until about age 15.
Since I don't know you two, I won't say "don't do it," but I would definitely advise caution.
Edit: Another thing comes to mind about safety, although not traffic-related: Our younger son (I don't remember about the older one) definitely could not handle nearly as much heat before puberty, as he did not sweat much. A ride in slightly hot weather was no fun for him and he would be unable to perform, and crying. Some parts of the coast-to-coast ride will definitely be hot.