I don't think making it a feat is the way to go. Then there is no way to limit it just to the smaller races and it could really get out of hand. I think maybe giving it the "longtooth" powers would maybe make the ability worth a +1 enchantment value.
Enhanced/Longtooth
This weapon when in the hands of a size Small or smaller user increases the damage die by 1 step as per the table in the DMG. For example a halfling using a short sword with this enchantment would deal 1d8 instead of 1d6. The weapon can also cut through wood or stone with ease. This allows the weapon to ignore the hardness of objects made of these substances.
Caster level: 9th; Prerequisite: Craft Magic Arms and Armor, creator must be size, Transmute Rock to Mud: Small or smaller; Market Price: +1