You might have to go to the special services check in counter land side in ATL in person to pay the fees for the cats.

I assume you've already checked the rules for bringing cats on board DL: stuff like health certificates, minimum age, whether they can share a container (hopefully yes since you're only allowed one animal container per passenger), making a reservation for the cats, which cabins and seats are allowed on which DL aircraft, etc.

There's also a Traveling with Pets forum here on FT that you might find useful.