I'm not sure, when all my kitties were born, we kept them in quite a high sided box with a wee makeshift door on it. We'd leave it open during the day but keep a close eye on the kitties and closed it up before going to bed so that we didn't have any accidents during the night or in the morning when we got up.
I'd say just let them have a wee wander but keep an eye on them. If mum doesn't like it she'll most likely pick them up and put them back herself anyway.
Maybe you could get a bigger box also, I don't know how Mum would feel about moving but our Doodles seemed ok with it. Every cat's different tho.
Don't rely on my advice, there's bound to be someone around who knows a lot more about this kind of thing