i think if young link is put in, it should be the OoT young link, AND adult link, with a master sword taunt-transform... that would be perty cool... moveset wise (or rather item wise) it would make sense to give young link the bombchus instead of bombs and obviously the boomerang should be exclusive to young link instead of adult link... item wise:
young link could use the masks, the magic beans, the slingshot, the boomerang, the deku stick, aaand i guess he had the owl, maybe...
adult link had the arrows (+ the magic arrows), the hammer, the biggoron sword, the hover boots (maybe can be like peach's hover, but shorter), the iron boots (maybe give armor on crouching, like he's put them on..?), the different shields, the suits, and i think the hookshot was the last thing...
...now, not counting majoras mask, this is the canon i think... other than these items, they BOTH have access to:
dekunuts (recommend to young)
bombs (recommend to adult)
bombchus (toy-like, so young)
the magic spells (adult)
and the lens of truth is the last item i can think of, which i guess could be a counter if you REALLY don't have enough here already xP
...but my idea is to give adult link the din's fire from OoT, and young link Nayru's love if it fits (to avoid too many magics going to one link)
the slingshot could go to young link, maybe with a stun, and the fairy bow and arrows to adult link (if possible, make fire and ice available, and only on fully charged)...