How tall do small kids need to be to be able to snow tube with an adult

My 4 year daughter was able to go on with me. To ride alone they need to be able to stop on there own.

Essentially, a child can ride on their own as long as they can touch the ground in order to stop. If your child cannot put their boots or shoes down to slow the tube they must ride in the lap of an adult.

It's 3 and up. No minimum height limit.