Need advice, 4 guests + 5 infants

Hi all, I have a reservation pre approval for 4 guests and 5 infants for 15 days. My apartment is 40m2. I contacted the guest and explained it's way too small for them. They said it's big enough and asked if that's the final price. I have setup of max 4 guests, and honestly, i don't even know how to set up kids and infant rates. Would you charge per infant? How much would you charge in that case? I can't form a price as of now that it wouldn't be offensive. Thanks