6/8 Foster's act petrified when meeting adopters.. They are so friendly around us. Advice?

I am fostering through my local shelter's found foster program a mama cat and her 7 babies. The kittens are now 5 months old, I've had them a while, and the adoption applications just finally started coming in. I had the first potential adopter come to my house yesterday evening to adopt one of the white kittens. These kitties have been so sweet, lovable, and friendly to everyone in my family.

The woman and her teen daughter walked into the room and every kitty except for the mama and one black kittan spent an entire hour cowering in the corners. I am so glad these adopters were so understanding after I explained they're really friendly with everyone and I was shocked, It must be the new smells on them and that they have 4 other cats at their house that were probably being smelled as well.

They decided to adopt one of the ones that was quite skittish after I explained his personality and how he behaved once he got comfortable. They said that they've previously adopted and have a cat that was like that, that took a month to warm up and he's now a great kitty so at least they were understanding but I am worried it will affect the others for future adopters..

Does anybody here have any advice on how to fix this? They are all just so so sweet but aparently uncomfortable with strangers and new smells I guess.