Do they have enough room in the "makeshift" brooder? There should be plenty of room for them to run around. A perch will help, mine love the perch. I guess it's possible the older chick is doing the damage but have you ever seen it attack and try and hurt the younger ones? If so, you may need to separate it.
Ironically, I just finished sewing up a duck. Seems it flew the pen and something got it - I'm guessing my German Shepherd puppy. It had two tears around neck and a patch of skin missing from under belly. The neck wounds were easy to sew up but the belly just didn't have enough skin there. They are amazing creatures though and can survive. If you think the other tear needs a stitch, go ahead and put one it. I find it heals better than gaping open.
Ironically, I just finished sewing up a duck. Seems it flew the pen and something got it - I'm guessing my German Shepherd puppy. It had two tears around neck and a patch of skin missing from under belly. The neck wounds were easy to sew up but the belly just didn't have enough skin there. They are amazing creatures though and can survive. If you think the other tear needs a stitch, go ahead and put one it. I find it heals better than gaping open.