I think that maybe you should start from some fundamental questions like why one should at all want to write in a functional programming language (immutable data, pure functions etc, and perhaps you have to learn it first by yourself).

If your child already understands some OOP concepts like objects, classes, you could approach it from that perspective, that is, functions are special case of classes (and then explain why we want this). The whole Clojure language on top of JVM is built on this idea, so why not.