If they feel start feeling adventurous, they could hop out of the box and get lost, which would be tragic. Do you think you'll be able to get a large or something to make a safer and long-term brooder?
Also, do they have water in there?
So sorry for your loss. :hugs
The older hen won't accept the chicks, and if the newer one didn't take them either, you'll have to raise them yourself. Do you have a tote or box to brood them in indoors or a place outdoors you can put them?