Try putting the kittens in a carrier and letting the 'big cat' sniff them without having to worry about the little fellas crawling all over her. Sometimes cats are intimidated by a group of little crawling kittens approaching them. By putting the kittens in a carrier, it allows the 'big cat' time to sniff and adjust without fear. See how that goes for a while, then maybe if she's ok with that, then try one kitten at a time and see what happens, but always supervise them!

Good luck.