When can they be let out to wander around?

Hi. My cat has kittens that will be 2 weeks old on Thursday, so they're 13 days old right now. All 4 of them have their eyes open and are feeding normally. Right now they are in a padded box that is too tall for them to crawl out of. They used to be in a box that only had 4" or so high walls, but they kept rolling out of it and at the time were too small to climb back in or even know to climb back in. My question is, when is the right time to move them to a shorter box (or modify the box I have) that they can crawl out of? I don't want to move them too soon and have the same problem as before, but I don't want to wait too long and stunt their development. Any ideas?

Call me simplistic...

But if the mamacat has been attentive so far, I'd trust that she can keep the kittens sufficiently corralled and decide when they should be allowed to roam. She'll scruff her kittens and carry them back to homebase if they wander too far away.

Interesting... never thought about it.
The barn cats have healthy , happy kittens, and there's a whole barn there for them to get lost in. They never do. It's so cute... they always seem to pick a hollow in the hay bales. Then they move to different hollows over time. Mama goes out hunting all the time and the kittens are always safe.

No idea how those Mamas do it... but they sure have complete control. And they never looked exhausted nor a hair out of place. Their kittens are always perfectly groomed, fed and quiet.
One tiny kitten wears ME out...

Is your box cardboard ?
Maybe find a bigger cardboard box... sit the one they're in ...inside the big box, and gently remove one side of the smaller box ?