I hope you have found a ride for your nephew and you. Last summer I traveled the Kettle Valley Railway in British Columbia (just outside of Penticton) with a 6 and 10 year old on their first bike touring experience. It was wonderful! It is an abandon railway so the grade is very gentle. Backcountry travel but lodges and lakes along the way. I've also ridden 300 miles of the Divide trail and think it could be a little much for a first timer. Check around for other Rail-to-Trail routes that travel through the backcountry - I'm now a huge fan after our experience in Canada! Contact me if you'd like any ideas.