There are way easier alternatives to explain Bitcoin to a child and make them understand how it works for the most part.  But in my opinion, this should not be a lesson.  They ask the questions, you give the right answer at the right time.  Kind of 'enforcing' Bitcoin onto children is a subject I have seen around here a few times in the last year and I am not saying you are planning to do this OP, but some people do it.

It would also be a big mistake to put the word 'thieves' next to Bitcoin when teaching your child how it works, much like pooya said.  They grow up hearing Bitcoin is dying, Bitcoin is used by criminals, Bitcoin is a bad thing et cetera.  All of this AND the most trust worthy person to them, their parents, taught the kid it has to do with thieves and magic spells too!  Not a good thing.

You would be surprised to see how much talent some people have to simplify the Bitcoin definition and how it works.  It is a hard thing to do, but possible.  But even then.  This still only teaches the very VERY basics of Bitcoin, nothing more.