Dimension will definately prevent germination of grass seed if Dimension is used before sowing the seed. Been there...done that.
But if you can get your new grass up to about 3 inches tall before applying the Dimension crabgrass control...and crabgrass has not yet sprouted...Risky, but it should work out OK.
I hope to try this on a bad lawn test site this year. Better test than using flower pots.