Next project is a dust bowl (or square as it may be). Do pullets take dust baths or just mature hens?
My girls started dustbathing as soon as I gave them a dirt clod to peck at! I think they were maybe a week or two old at that point. It's super cute at that age. They flop and fluff around, then just lay contentedly on their sides making happy little trill noises.
Honestly, with your setup I would probably just fill it in with dirt the the level of the 2x4s. The girls really enjoy scratching around in the dirt, looking for bugs and rocks, and mine haven't been very happy in the wire-bottomed tractor because it keeps them from doing those normal chicken behaviours. If you just give them a few inches of dirt they can get that satisfaction while still keeping them safe from burrowing predators. Then they can make their own little dustbathing areas as well. Just a thought.
Such hungry little things! I always read to elevate their feeder to the same level as their backs. If they haven't already, soon they will also try to scratch at the food (it seems so weird, they are too little to act like adult chickens!) and knock even more out. I used bricks from the garden to elevate it in the brooder, but really anything will do.