i'm sorry that you didn't take the mother. imo, they all would be better off if they nursed from her. even though she's feral, she could have lived outside. eventually she would have trusted you enough to be 'your' cat. many feral cats become tame even though they prefer never to live indoors. the kittens would have been indoor/oudoor cats. she should also be fixed up so she doesn't keep producing kittens, no matter how cute they are. sometimes a vet will discount or even not charge for the service. if not, find one that will, through a no-kill shelter.
good luck with those adorable babies. how old are they? remember to make them independent when they're about 4-6 weeks old, even though they stay with you. that's the natural order of kittens and mommies.