You have to grab the CV and pull the shaft out towards the hub. Make sure this is what your son is doing. Not sure how big he is. Perhaps it will require an adult's muscle to put it out enough? It will slide out slightly and you should be able to install those snap rings.

They're a Pain in the <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> to do but they work well enough. <img src="/forums/images/graemlins/kewl.gif" alt="" />