If I buy Microsoft Points and buy a map pack, can other users of my xbox use them? I suppose the question is, who can access downloaded purchased content on a console, is it only on the ID that purchased the points, or is it all users on the console. Any guidance greatly appreciated. I want to buy MS Points with my Live ID, but my son wants to pick the content to download to be used under his id. What's the best way? I buy online, or he buys from a shop?
To answer the first half of your question: as mentioned in this answer, anyone can play DLC on the console on which you purchased it. (Because it is also linked to your XBL account, you can also play it on any console.)
I know there is some way to set up some kind of "family" setup so you can control what your children buy and play, but I don't know any details on that. Hopefully someone else can fill in those blanks.