Hi Steve and welcome. It's my OH that's the grass expert really not me, but he doesn't come on here so I'll reply!
You don't say how 'new' the grass is, is it seed just sprouting or turf? Anyway, I wouldn't use weed and feed yet if its new grass, presumably the weeds were in the soil already so I would maybe try pulling them up, making sure you get the root especially with dandelions, depending how established the grass is you could then fill with something like patch magic?
I how this helps a bit. I'll ask OH later too and may post again,but give us a clue about the grass, maybe a picture?