If they are able to move around well let them out in a carpeted area (or if you have hardwood floors, put a towel down so they will have some traction and keep warm) each feeding. That will not only give them a chance to explore and develope those muscles, it will also help get their bowels moving.
I don't know that there is just one answer for how to do this. except that I definitely would not let them out to just run around the whole house at this age. It's a matter of balancing different needs -- giving them some opportunity for exercise and physical and mental stimulation, while also keeping them safe, healthy , warm ..... and they are right around the age that they'll soon be starting to pee and poop on their own without stimulation. but won't yet have much control. so another issue is you will have to deal with them having accidents which will need to be cleaned up promptly and thoroughly, to discourage them from being attracted back to that spot.
If the box is small and they don't have room to play and move around and exercise much, you can definitely let them out at least some of the time , either into a larger sort of playpen / cage etc. or into a kitten proofed room. with places like under furniture blocked off . Or you can decide to just let them out in their nursery room for play periods when you're able to watch.
Or what I do from around 3 wks is a combination of the two . I have a sort of pop up playpen for them.
so it's not like a very small box, and they do have some room to play . but also some of the time they can run around the whole room.
& I also start sometimes just taking one kitten out for one-on-one socialization. ( I'm talking about kittens who have their mom. so they're not bottle babies who would already be getting some socialization to humans at every feeding.)
Got to agree with Maewkaew about the playpen she added the link for. We had two of them and sewed them together and left our kittens in there when they were young, it was also useful for containing the kittens when we weren't at home.