Advice please on housing kittens

I've taken in a litter of 5 newborns and their mom. They are one week old, and I am going to care for them until they are old enough to be adopted. Any advice on what to house them in when they start walking around? I want to keep them safe, but also need the mom to have access to them. I thought about a travel play pen, but then mom can't get to them.

I have a puppy play pen that someone gave me, that I moved the kittens to when they started moving around more. It's about 2 ft. high and mom can still jump in and out (she can do it on her own anyway, but I still have a chair on the outside and a crate on the inside for her to jump in and out. Inside the pen is their litter box, water, and their crate that they were born in full of blankets. I have to put something along the bottom, otherwise the smarter ones will squeeze through the bars so they can run and play in the rest of the room. My kittens are 4 weeks old, and in about 2 weeks I know they'll be climbing out of it. Our bedroom is completely kitten safe, so they'll be fine just running around in here when they're escaping their cage.