I would like to suggest the strategy I always use in these kinds of situations:  listen 95%; talk 5%.

"So . . . you've been working on this project.  Teach this subject to me.  What is the first thing I need to know in order to understand where you are going with this."

In other words, don't mentor--be mentored.  Become a student of the subject that the child is trying to master.

This assumes, of course, that the kids involved are self-motivated, naturally curious, and able to muster rigorous, creative, and/or analytical thought.

Herring405