Second things, what is the room they're in like? Very hot? Very cold? Jumps up and down all day long? Very light?
After changing their water, your next priority needs to be ensuring that their environment is as stable as possible- the room needs to not be heated or cooled unless the tech's on 24/7. Next, you need to ensure that the water that you're using for changes (Which need to be happening AT LEAST every 24 hours, if not more often) is the same temperature as the old water. I do this by floating the babies' boxes on top of the adult tank and drawing my water from there.
Next, diet- Are you feeding bbs? If so, good food choice, but remember you only have a few hours after feeding before they start dying off and causing ammonia spikes, which affect babies much more severely than adults or juvies, so a water change is essential within that window (If you have fish housed separately, they will more than happily eat any leftover wigglers) . If you can get your hands on live daphnia or blackworms, they are a good food if you can't be around to do a water change after dinnertime.
Next, have a read through this:
Caudata Culture Articles - Why Larvae Die whilst in this case it looks like something environmental has gone wrong, it may help you isolate any other contributing factors (For instance, was it the mother's first batch of eggs? Are the parents in good health?) so the situation can be avoided.
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