Thanks for your help.
Can I buy it for my account (as I'm the 'adult' account) and allow my son's account to use it
You will need to put the Xbox Live subscription on your son's account. I think the first month is free so you can trial it for a month first to check your signal is ok etc before shelling out £40.
HastingsSeagull I totally see what you mean. I think I'll just get a 3 month membership while its all still a novelty. Anyway, he can always play me if he can't go online. Took me a week to beat him after he got it for Xmas. Scoring that winning goal was bliss (not that I'm a competitive dad or anything)
And dont shell out £40 !!!!!! £25 on ebay, buy from a reputable seller and all you need is the code. I have done it dozens of times for me and my sons(10) xbox live accounts. His is set up with parental control that requires you to authorise friend requests and mutes other players by default. Its not hard at all !