I'm not sure what you're asking. Why your platies died, or what you can do about your fry?
For the deaths, we need to know your water parameters (PH, Ammonia, Nitrite, Nitrate and temperature most basically). What is the male/female ratio of your platies? Also I looked at your profile and the only fish I can think of that would bother platies is the ruby shark. I don't know anything about baby whales.
The fry, if you want to keep them, should be moved into a small tank with java moss and other plants to provide places to hide and infusoria to eat. You can feed finely crushed flake if the fry are big enough. Livebearer fry are born bigger than others, so they may be able to eat this soon. If you don't have a tank, you can use a clean bucket, just remember to use a heater and filter. The water should be partially changed every day or every other day.
If you don't want to keep the fry, they will get eaten in the tank without any interference.
HTH
