I used Roundup in the early Spring this year for the dandelions.

I used it when rain was forecast in 2 or 3 days; and I did not let the cats out in the yard until AFTER the rain.

It was enough time for the Roundup to work and kill the weeds. I think it mentions you need 24 hours of no rain for it to work, but I don't remember.

Better to be safe, good for you for asking!

If the cats are in a new home, you really don't want them outside for a bit anyway. You need them inside setting their scent and understanding that THIS is now home. Good luck!