Can you somehow make like a small fenced in area right outside the crate for them? That way the kittens can move around a bit, but still be in mama's sight. A couple of times, I've made one by cutting down a large cardboard box and lining it up with the nest. Keep the height of the box a couple inches above what you think the kittens can handle, but still easy for the mom to get out. Though in another week, you'll be limited in what you can do to contain them. Hopefully the mama will be ready for the kittens to explore by then.