now what to do with the feral cats?

Cat flaps are fine, but at the time I wanted the cats to get used to being inside the house, not outside

I am very glad you ultimately decided to do what is IMHO the best for this cat family. I am sorry that your little ones may be feeling disappointed. Perhaps you can still befriend the cats by feeding them but do not do things like trying to pick them up while they are eating etc.

Perhaps sitting down near their hang out and playing with a cat-appropriate ball or toy, without paying attention to the cats might pique their interest. You may have to do this more than once. Cats like to approach people on their own terms and spend a lot of time checking you out and evaluating you. No sudden moves or loud voices.

Good luck!