My cat trusts me, so why has she hidden her young?

Cats can really change when they become mothers. Their young are their first priority - just because she doesn't bring the kittens near the house doesn't mean she doesn't trust you, she just might have other reasons. Maybe she has found somewhere safe and warm for them, and feels that is the best place for them. Once they get older they'll have more independence and the mother will move them around more. Just trust her mothering skills, and let her raise the kittens - I'm sure she's capable.