What is this plant?
These look like suckers growing from the roots of the tree, so do NOT spray them with a weed killer, or it could also kill the tree. Callery pear trees and crabapple trees often put up root suckers and sprouts under the tree. Is it one of those two types of tree? It definitely is NOT poison ivy or any other plant that would cause dermatitis problems. The only safe "solution" is to keep cutting them off at ground level with a pruners, or else just keep them mowed short.