Fertilome Weed Out Lawn Weed Killer with Trimec would work for your needs. It will control Aster and other broadleaf weeds without harming most lawn areas. While you cannot apply to newly seeded lawns, new seed can be sown 3-4 weeks after you apply this product. The label for this product will specify how much to use per area according to what turf you are applying over that you are trying to keep. You can find a link to the product and the product label on our website here: http://www.domyownpestcontrol.com/fertilome-weedout-lawn-weed-killer-with-trimec-p-1543.html
It is safe for pets and children to re enter the treated area, after the treated area has dried. This will take approx. 4-6 hours.