I did this on a tandem with my kids when they were 7 and 8 .Even though they have done several state rides they weren't ready to solo that far at 9 and 10 . It's not only the ride , it's the mornings you get up ane it's 45 degrees and raining and your stuff is all wet . Or the morning you get up and it's 26 degrees and it is hard enough for an adult to get motivated ,and that happens at times too . Kids usually don't have the mental toughness . I think you would end up pushing them way beyond what they would want , and that might ruin them for the future .