Just trot down to the local BTC ATM and feed fiat into the slot.
My nearest ATM allows purchases as low as $5 (smallest Canadian banknote).
That should be within range of any kid's allowance.
Lol, look for a BTC ATM & lose money on fees. Then realize that the smallest amount MS allows to be added is $10.
Or walk to the nearest store and buy
Don't even bother.. it's like trying to teach a dog algebra. Only a bitcoin true believer can actually imagine that a teenager would go through pain staking lengths, and cough up fees so that he could get bitcoins, only to trade them online for an xbox gift card.
Rather than buy one online with mom and dad's credit card and pay them back in cash... or you know... just walk to the store and buy a top up card off the shelf without any hassle or bullshit.