You sound like a pretty experienced aquarist, so I'm sure you understand my teasing.

If well fed and with some shelter (plants, plastic plants, lock rocks, unraveled plastic pot scrubbers and so on) there should be some guppy, platy and mollie fry surviving. If there are other more carnivorous fishes there, they may have squeezed the guppies into their mouths.

Oversized female guppies seem to be a hot topic right now - Why is this happening? Guppies/Mollies  Guppies Losing Their Color?   How many fish?   Pregnant Female Aborting Babies?  

It can be that yours still has fry to drop. Guppies seem to drop them pretty much at once. (Platies may do that or string them out over several days.) Of course, the fish have never read the book and do what they "want."

It could be, if the fish was on a heavy blackworm diet, that she was simply obese. That's still not particularly good news.

Too many guppies from commercial (and even hobbyist) channels may also be infested with either worms or bacterial infections. If the fish is far enough gone, it may be hard for you to help her. (See Quicklinks on Camallanus or bacterial sitautions relating to Dropsy/Bloat or Columnaris for some similar situations.)

If you have successfully kept oscars, you probably  could teach guppy people a lot about nitrogen cycles and keeping the organic waste content down in tanks. It sounds like the malady effecting your female guppy could have been inherited from the shop.

You probably already have watched her to make sure no companions are stressing her. As simplistic as it sounds, if you are keeping an eye for her health and more fry, making sure everyone in the tank is well fed (I do add rinsed defrosted foods or even worms in small jars so there are "tasty" alternatives to guppy fry) and that you are changing the water like you would in a cichlid tank, your kids should find fry.

I did 90+ % water changes on some quarantine "containers" yesterday. 60-70% of the water was from a healthy established tank I use as my "start-up" tank. The rest was from a heated "reservoir" which was holding tap water which has been "seasoning" for the last 10 days. The killies which were showing velvet are looking a lot better today (I did add acriflavin and darken the place as well) and the rainbowfish, one of whom was clamping, are also looking better. They will get the same kind of water changes again tonight.

The fish in my set-up tank are not hurt by having 40-50% of their water changed daily either. ;)

One last thought, I know that appeals from children are hard to resist. But you don't have the goldfish in with the livebearers, do you? ;)