We have our bikes and we travel around 70 to 100km a day. Last weekend we did 77km the first day, 95km the second day. This equates to 6-8 hours on the saddle.
He found a passion for it and he keeps asking me to do such trips. What's helping also is that we live in a place with a lot of bike infrastructure, so we are always secure.
Your 10yo does 70km+ bike trips???? That’s incredible! Also makes me feel less than stellar about my own bike rides :P
I'm not a fit person (at least 20kg overweight) but I have strong legs which help me a lot. It seems my son inherited my legs, he powers through without complaining.
I think next year I'll try a week long 700km trip with him, I would like to follow the Rhine River up to the sea.
I hear the Moselle river valley is nice for bike tours, though not quite as long.