One thing that small boys from Catholic families sometimes do is to play at saying Mass. One of my sisters actually managed to get a boy's Mass kit from somewhere in the US complete with vestments so I must find the link for whoever supplies those things. Our Father's House has a couple of kits and a few months ago the Curt Jester posted on a Mass Kit thoughtfully made of foam so as not to disturb reverent environments.
I'm wondering if there are any advanced versions because a mother told me this morning that she went up to the boys' bedroom to check on them and found a lot of their Mass things on the floor; when she asked them why, they said "It's all right Mummy, we're setting up for Low Mass." Last Sunday, apparently, they were agreeing on who should be Deacon and Subdeacon. My MC and I got a little carried away in wondering mischievously whether we should try to obtain folded chasubles for them in time for Advent :-)
Before you ask, let us not forget the girls. Where can we get some small versions of the habit of the Franciscan Sisters of the Immaculate?