Thanks, JustClucky, I will keep an even-closer eye on them. That must have been horrible!
It's been about 24 hours and they all seem perfectly happy so far. We did have some poopy butts this morning, more than usual (perhaps because the down is fluffier? I don't know.) I treated with a careful Q-tip application of olive oil around the vent, as I have seen recommended here. I guess it helps the poop to slip past the down. One of my Brahmas has taken offense and was plucking out the down around her vent, but whether to get rid of the poop there or the olive oil, I can't tell. She seems to have stopped now, and is eating and drinking as normal. The other girls seems to be leaving her little red bum alone, praise be. Also, their feet seem to be cleaner, even the Brahmas.
I will say that there are a couple of things I would have done differently. I would have prepped the brooder with tons of time to spare, because when the Play Sand comes out of the bag, it is a little damp. I would have spread a bag at a time, to allow it to dry in between bags. There is dust with sand when I clean the poops, but it isn't bothering my lungs the way the pine shavings did. Perhaps because it's non-organic. And the poops are very easy to clean out, I must say.
As it is now, they all seem very happy scratching for their crumbles and nestling down in the (now) warm sand. So far, so good!