There is a ton more selection in 12b. The key is to go with the lightest ammunition possible. Fiocchi Training loads, Win AA Featherlites or RST Maxilite will be much more pleasant for a youngster. The extractor gun with 26" barrels is a great idea. If necessary, cut down the LOP and keep the cut off piece for re-installation later. Might add a good soft recoil pad at first as well. If you do go with a double, I would load only one barrel for him at a time until the kiddo gets more than enough experience to avoid an accidental discharge of the second barrel.
