Welcome to the flock! I personally use deep litter in my run, IMO, that is the most low maintenance way to go. Sand will need regular scooping of the poop to keep it odor free (ish). You will want some more boards along the bottom to hold litter in as you add more. Mix in pine shavings, leaves, bark mulch, etc. The chickens turn it over and mix in the poop, as it breaks down, it composts. For a small run like that, I would just cover it with hardware cloth, and throw a tarp over it, for shade. Since you can't free range, I think at some point you will want to enlarge it, but it's fine for now, for young pullets. You will figure that out as you go, and gain experience, to know what works and what won't, for your.own circumstances. You want to feed a chick food, or all flock ( flock raiser) feed, until they start laying.