I think it's better than okay...I think it's wonderful Have you spoken to anyone from the shelter? Chances are good that they have live catch cages available or know where to borrow them. There are groups and shelters here who use/loan the traps for feral cat population control. The cats are trapped and then spayed/neutered and returned to the colony.
You're very fortunate to have a no-kill shelter and even moreso to have one that isn't at capacity. If you've talked to the shelter they may have told you that the cat and kittens may not be adoptable. It isn't easy to "humanize" a feral or semi-feral kitten and next to impossible to do it w/ a grown cat. Whatever happens, I think you're doing a very good thing. Far more than most would do. Best of luck to you and the kittens