Hello Donna, happy to help.

I really can't tell what tree this is from the photos but it's likely one of your trees that's seeding or suckering these seedlings.  That second set of leaves at the top are the true leaves of the tree.  So check their form against the leaves of the trees in your area.  Knowing what kind of tree it is will help determine what you're dealing with.

When you pull them, are you getting the entire small root?  If not and you see that what came from below ground is broken off, it may be from a suckering tree.

Either way, mowing will take care of most of the problem.  All of it if the trees are not suckering.  So save your energy and concentrate on the seedlings that don't get mowed like in your gardens.  Mowing will certainly keep the suckering ones under control but it's best to remove the suckering root when pulling up the seedlings in areas that are not mowed.

As for identifying it.  If you can narrow it down to a fully grown tree in your area, photograph its form, trunk (bark), detailed branch with leaves, time and color of bloom if you see any and reply to this thread with that information and I'll see if I can identify it.

Good-luck!