That looks more like violets than plantain (too high to be plantain, which is about a half-inch tall rosette at this point in the year, assuming that you're in the Northern Hemisphere). The first photo also shows something with a fern-like leaf (Queen Anne's Lace maybe?). The grass itself looks old enough to walk on, so if you're careful you can try to manually remove the violets, but you'll have a heck of job trying to eradicate them - no chemicals that I know of will kill violets (and an herbicide at this time is likely to harm the lawngrass).
EDIT - Well, the third picture shows that they're not violets. The venation on the leaves in that photo shows, also, that the weeds are not plantain either (plantain leaves don't have a central vein). They're certainly quick growers, which has me thinking "annual". Because they were already in the soil when you reseeded it, I also think they're either a common weed in the PNW or native to the area.
Whether annual or perennial, keep those plants from flowering and setting seed. I would also try an herbicide in a few weeks (probably something like Weed-B-Gone) if you don't want to hand-pull them.