Mass Die Off

So yesterday we caught my 4 year old boy sharing his fig newtons with our fish. I netted as much as I could and did a large water change of about 50% to try and siphon out the rest.
This morning I found both of my snails and one Cory dead, the water was cloudy so I took a sample and while the tests developed I did another water change, this time I only left about 4" of water in the bottom of the tank.
Water tests came back with 0 ammonia and nitrite and 20ppm nitrate. I temperature matched the new water and treated it with stress coat+.
So as of now I've lost all of my snails, 3 corys, all of my platys, all but 4 fry and my sword tails are gasping at the surface.
Can anyone suggest anything more I can do o save these fish or is it a lost cause?

More water changes, clean the filters as some maybe trapped in there.
 
Sorry to hear about that. It's horrible.

Ty, how long should I wait between water changes to avoid shocking the already stressed fish? I've done 2 already today and rinsed the filter media in tank water but the water seems to actually be getting more cloudy. Lost another Cory.

I wouldn't worry about stressing the fish with water changes considering that they keep dying. Do as many and as much is needed to stop that.  I'd test the water again too.
There's something that's in the water which is toxic to these fish.

So I've lost 7 fish today and all of my snails. Only survivors are 2 sword tails and 1 Cory sterbai, and amazingly a few fry. with how they're swimming/breathing I'm not very confident that they will survive the night.
I've done 3 large water changes in about 9 hours with apparently no effect. After each wc the water clouds over to a milky white within about an hour.

I'm at a complete loss as to what to do, I guess I will just have to wait til morning to find out if I have any fish left.

Take out all fish from the tank. Put them in a container with the filters, heater only.
Then drain that tank, wash the gravel and decor. Fill up with fresh dechlorinated and temperature matched water and acclimate the fish back to the tank. You can acclimate them in the container they are in by removing let's say 50% of their water and fill it up with fresh dechlorinated and temperature matched water as well. Then net and place in the tank after putting the filters back in.
 
Monitor the tank params for a while as washing the substrate may cause some ammonia rise depending on bacs but at this stage there's seemingly something left in the tank,between the substrate causing the fish deaths.

My remaining fish died while I was attempting to clean the tank. I don't understand what could've completely devistated all of my fish in a matter of a day. I did notice that my male sword tails fins seemed to be disintegrating before he died. Only showed up in the last hour. I'll post before and after pic of him, maybe it will help solve this. I'm going to tear down and sterilise the tank tomorrow, not sure if I want to try restocking if I can't even determine how I managed to kill these apparently healthy fish so quickly.
Before:
After

O, gosh, sorry about that.
It's probably something from the food that went into the tank.
Just clean out everything, not sure it will need sterilizing but if you want to go that route and recycle the tank it's up to you.
I don't know what to say.

Really sorry about your fish

Wow, I'm sorry that all went so badly.