Can not see your pictures, the URL would not open.

The foliage reminds me of a buttercup (Ranunculus). What part of the country are you in?

Hi,
It could also be a weedy geranium. The foliage of it and buttercup are very similar. The same products will kill both too so just look for them listed on a lawn weed killer product.
Remy

Its fairly common to get weeds in a newly seeded lawn. I would just wait for the grass to germinate and focus on growing healthy grass. Eventually the grass should outcomplete the weeds. Especially if you mow high and water deeply and infrequently.