Here is a You Tube regarding the Christmas house.
Regarding volunteering, the Salvation Army is right on Airport Road, very close to where Uncle Harry's used to be. (Senior moment, can't remember what it's called now) SA should know how to contact other organizations. I know there is an organization on island that is called Island Gems that helps out local people, but not sure how to contact them.
Regarding other Christmas activities, I know that Mary's Boon used to have a Christmas Carolling event there on Christmas Eve, and they usually have some kind of a show in Maho.
Regarding the actual day, I know a lot of restaurants are closed that day. Christmas should be about being with your family anyway, so make plans to have a meal in that day, and give time off to all the hard working restaurant workers. And for kids, even on SXM, I know that they would still want a TREE, even if there's not much underneath it. They do sell real trees on island, but you'd have to figure out ornaments. You might want to think about bringing a real small, pre-decorated tree for them.
Regarding mass, we are not Catholic, but I know they do have Christmas Eve mass on island. There is a Catholic church in Grand Case and one in P'burg, pretty sure. You might check with them, earlier in the week. Enjoy!
.
