The way to prevent weed eater damage on young trees is to keep the revolving string away from the bark and cambium layer of the tree. Stay a couple of inches away from the trunk of your trees.
I have seen more young trees killed by weedeaters than any other cause. Death by weedeater is a common problem with inexperienced gardeners and new homeowners.
It is better to hand clip the blades of grass around the base of the tree than to take a chance on damaging your trees.