We did this when they were 3 weeks old as they were climbing out of the bedroom cupboard that was their nest and we thought it was unsafe if then Pixie - the mum cat got unsettled and moved them onto the bed - which is very high and they wondered off the end!
We made a bigger nest/area for them in the living room and surrounded it with the side of our sofa and a couple of foot stalls, when we went to move them we made sure pixie was out the room and gathered the kits in a box and took them straight into their new area, then let pixie in, but kept the living room door shut. We really thought she was going to try and move them but she didn't! She settled down with them straightaway and is very happy there! A week later they had found out they could climb out but it's fine and at least a lot safer, now at just over 5 weeks old they come out and play they tend you still go back to the nest to sleep and feed, although not so often, off mum.
Good luck, but I recommend 3/4 weeks.