As someone has said before heat lamp and maybe an area where it was covered around the pen so it would keep some heat in.
Also is the feed containers held down by anything? If not you might wanna get something to keep them held down because ducklings like to knock things over.
Its to hot here for a heat lamp. But I have one just incase. My mom said that they dont need one, but I said thats not what BYC says! And shes like ok but they need togo outside and im like thats not what BYC says either so I get to have them inside till she gets straw next week.
If they're in the house and you have air conditioning, they'll need the heat lamp. They might not spend much time under it but it's better safe than sorry. You could brood them outside but I wouldn't do it in that cage. It would be too hard to maintain temps.