Your plants appear to be common groundsel seedlings, also known as ragwort. This weed is propagated by seed, which explains their sudden appearance in your lawn. It is most commonly found on moist, nutrient-rich soil and is most troublesome in spring and fall when conditions are cool and wet. It tends to die during extended hot, dry periods.
It is important to eradicate these weeds before they begin to bloom and then produce seeds (which can occur from April until October). Hand pulling is one option for removal if there are only a few plants.
For chemical eradication, look for a broadleaf weedkiller that lists groundsel on the label as a plant that it kills. Be aware that opened flowers can develop fully mature seed after plants have been killed by cultivation or herbicides.