The doctor might be concerned about the Tazorac causing a stinging sensation that can be rather unpleasant, especially for a child. However, call him up, or wait 'til you visit again and ask WHY he wants you to wash it off. If what he's concerned about hasn't been a problem, then you might be ok. Just be sure to ask him or her first!
I am female, and was born in 1972 with Lamellar Ichthyosis.