When my chicks outgrew their brooder, and it was still too cold to put them outside, I just put an addition on their brooder. My chicks were in large cardboard boxes so this was easy to do by connecting another box on the end of the first one and then cutting out a "door" between the 2 boxes. they loved their addition.
As for when they will be able to get out....it will be soon
If you have an extra window screen hanging around you could put that over the top to keep them in.
As for when they will be able to get out....it will be soon
