This is a great way for your child to really get to know a piece they are learning. It also allows you to spend some really quality time together with no chance of criticism.
Get into position on the instrument just as though it is a lesson with the real teacher. Your child will take the role of teacher and you the student.
Have your child explain how to play the piece. Get them to describe what the notes are, how the rhythms should be played and anything else they can think of.
You can also take this a stage further. Do not make any assumptions. Take everything they say literally even if you know it to be wrong. Be a little ‘awkward’ and get them to explain really clearly what you should do.
Giving a lesson like this will really make your child think. If they are clear in their mind what is required when they go back to the instrument the benefits will be clear.