Certified GKH Gardening Expert
Those mushrooms are doing their best to repair that soil! Do not remove them, as they are removing the left overs of the tree, the disease that killed it, and the disease that is killing the grass. Removing the mushrooms will further the problem. Mulching the area can facilitate the process, giving the mushrooms more to work with so that they may remove the rest of the contamination in the area.
Should you still choose to eliminate them from your lawn, then this article will help: