They are about the size of a thumb.
They look like Norway Maple seedlings, if so get rid of em!!!!!
While the pictures are too fuzzy to properly identify what it is they are not Norway Maple seedlings. The leaves of the Norway Maple look just like the leaves on the adult tree, at least in my yard. If simply mowing does not eliminate these, hand pulling is the next best means of elimination.
When the maple seedlings around here open their first set of leaves they are a bit elongated and not as broad as the mature tree. It may not be a norway, but it sure looks like a maple seedling of somekind to me. I pull em when they are in the beds and mow when they're in the lawn. Had six mature 50-60 ft trees removed from the property last year, they were old and dropping branches, and produced tons and tons of seeds!