There's only one such line. In general it's a good idea to reset the transform before clearing as the (0, 0, w, h) rectangle may not cover the whole canvas anymore. In this toy example there obviously isn't another transform so it's without function.
Wonderfurl! It makes the kid see how a small algorithm saves time AND produces almost the same results as hand drawn animation. I have to try this myself with my own kids.
Absolutely! Assuming your 5 year old is some kind of savant. Otherwise, I can't imagine a kid actually understanding what is going on with a script like this.