I use small bark nuggets or shreds on the soil surface to reduce humidity. They also help support spindly stems without keeping them wet. I screen the bark myself to remove fibers and dust, which would wick water and keep the air near the stems humid.
My first thought when I saw those seedlings was "lights are not bright enough". If you can shoehorn one more CFL in there, like a spotlight bulb, aim it at just one tray and see if that tray comes up much less leggy. The comparison makes it a controlled experiment. (This might be an experiment for next year, if the spindly ones survive.)
They might also be too warm. Brassicas would prefer cool air after they emerge - maybe 60-65F. They would prefer 60F to 90F!
I agree that running a small fan intermittently will strengthen the stems. Even if it is just a few minutes a few times per day. Anything that makes them wave around a little will strengthen the stems.
