I would talk to the owners once again and explain that this kitten should always have access to their house and about the vacs/spaying operation she needs before she is let outside.

Sometimes people just need to be educated.

If they are not forthcoming, tell them that if you see the young kitten unsupervised outside again, you'll be taking her to the local shelter.

But it is only fair to give them a chance to be educated, who knows they could learn to be good cat owners.

If you have tried to educate the owners and they let her out again, they have been fairly warned. Take her.