Help. My babies are dying.
Likes to me like root rot from to much water.
Just my opinion.
I agree with Will. Root/stem/trunk rot from too much water, very likely pythium. It looks like the rockwool cube in the middle is getting or staying too wet. If it is in some type of DWC – the water level under the plant, under the hydroton clay pebbles might be too high? If it is in some type of flood and drain table it might be flooding too often or not draining properly.
In the future. Try watering around the rockwool or bottom feed if in Propagation dome. Never continuously dump water on the rockwoolo with the tiny plant being drenched. Use the rockwool as a buffer from the damp medium. IMO