Congratulations!?!?!? I am not ever going to encourage anyone to transport a pet in the cargo hold, I have lost two very precious little ones like that. But I will share with you that you CAN purchase a ticket and put them in that seat. As long as you stack the carriers and lash them securely together, the airlines look the other way. They just want the space used to be paid for.
I one carried FOUR puppy carriers with FIVE pups like this from California to Texas. It was a handful, and I had help at both ends, but it worked. I actually had a collapsable luggage trolley that I stacked them on for loading and unloading.
With so many babies, it seems worth the price of a ticket and the time negotiating for the arrangement. Unless you would enjoy a nice long road trip?
The more I learn, the more I realize how little I know