My 10 year old daughter can go 50 miles on her MTB and she is OK. Our longest was 64 miles. But there was a bit of a learning curve.
She wears child bike shorts and bike jersey. (Look like a biker = ride like a biker)
Her seat is adjusted for height and saddle fit.
She learned to spin not grind gears.
I changed her MTB tires to narrow slicks with max tire pressure
Her front fork in almost max stiffness.
We stop for all ice cream.
We rest at least every half hour for water and a placebo bar
We go for distance, not land speed records.
I carry all the water and stuff (dolls included), she has a few inches in her bottle for looks.
I let her lead, so she is not chasing the pace.
We take a lot of pictures.
Given a good attitude and encouragement, I think she could ride a century.