I have done a few tours my my eldest daughter and one with both daughters (who were 9 and 5 last Summer) on a tandem + trailercycle. So the speed was fairly much my speed.
Still, except on "perfect" terrain, riding 80-100 km (50-60 miles) took close to 7-8 hours. Add a couple of hours when riding through mountains. If your son rides his bike, you may need to ride two blocks of 25-30 miles rather than 50 miles in one shot.
Apart from that, we stopped once or twice at playgrounds (which my city kids prefer to sleeping by a lake), picked nice rocks along the road. I generally like to stop around 4-5 pm at a campground so the kids have some time to swim and play in the sand with other children.