Being that the lawn is newly seeded, I wouldn't worry about the weeds just now. As you stated, the grass is too young to handle the foot traffic. Some of the weeds are probably annual and will die out this summer anyway. Right now, concentrate on getting the grass growing as thick as possible. That will eliminate some of the weed problem; the rest you can tackle later.

According to the label, after the 3rd mowing the grass should be able to take using Weed-b-Gone. It also says you can reseed after three weeks, so you should be OK. The only issue is the heat. The grass is stressed in the summer and the weed killer further stresses it. I'd suggest spot spraying - just apply it to the weeds you see. Capt Tom

Did you roll the seed down? Did you water 3x per day for 3 weeks while the seed was fresh on the ground? Where do you live? What kind of seed did you use?

True, maybe not broadleaf. I jumped the gun, assuming the photo showed a plant larger than grass that had leaves ready to open. There's no scale to the image, like a quarter. A photo of the lawn might answer the question. It's hard to say what to do about it without seeing anything. If there are individual shoots here and there I'd leave them be and let the KBG grow undisturbed. If it is a carpet of weeds that might require some action. What state are you in? I assume it is northern since you seeded KBG. In most areas of the country this is the wrong time to seed a lawn. Unless you are in Canada or PNW it is likely that the new lawn will die out in July. If that happens then you will have an opportunity to prep the soil properly before reseeding. Is it a large area?