My adult cat had kittens- when is it safe to introduce them to my 4 month old kitten?

She had kittens and I've been keeping her in a room with the door shut so he can't get in, but then Freya (momma cat) can't get out either. When is it safe to leave the door open so both cats can go in or out as they like? Kittens are 2-3 weeks old right now.