I’d say you have reason to be concerned Sherry. All the growth action takes place in the bark of the tree. The wood inside is supporting the tree but the wood is kind of like our fingernails. The transfer of sap containing moisture and nutrients flows up through the outer layer, the bark. If the bark is removed all the way around the trunk then that flow stops and the tree will die rather quickly. Obviously you want to stop mowing so close to the trees so you’ll stop damaging the bark. Perhaps removing the grass and mulching around the trees would be a good idea so you won’t have to mow right up to the trunks. Since you don’t mention any od the trees turning brown or dying I’ll assume that the damage you’ve done so far isn’t fatal. Take a close look at the bark to see if you’d scraped the bark off all the way around the trunk. Chances are you’ve damaged one side but there is still bark on the other. The remaining bark has takenm up the task of sending nutrients and moisture up to the branches above. The damaged area will gradually recover and grow bark all around the trunk. You could get some Tree Wrap, a papery material that comes on a roll that you wind up the trunk. This will help protect the bark as it recovers and help prevent any future nicks from happening.