one way is you can separate them and feed them powdered flakes, liquid fry food, crushed algae tablets, large infusoria, bbs when they get larger, etc. (i.e. small versions of foods that you would feed adult platies.) feed them tiny amounts quite frequently (maybe 5 times a day if possible) and remove uneaten food with a siphon or turkey baster. depending on the size of their little separate tank, change the water more frequently and when they get to a size that they are no longer threatened by the fish in your main tank, add them one by one back into the tank. usually at about an inch in length they are ok, again depending on their tankmates.
alternatively, you can just help them to grow and survive naturally in your tank. what else is in your tank? if the fish and creatures in your tank aren't especially predatory, you can help them survive by putting java moss, plants and a heap of large marbles in the tank for them to hide in. then you can use a turkey baster to squirt the foods mentioned above down to where they are.
I used to catch them and put them into a breeding tank so i could keep an eye on them and also feed them crushed flakes and the occasional bloodworm.. But thats when i wanted to stock my tank a little more, but now, im going to end up getting overrun if i carry on catching them!!
So now i allow them to fend for themselves and hide in the bushy plants i have in the corner of my tank, my larger fish tend to stay away as they cant swim through them as easy as the others can..
I have mine separated in another tank, theres around 60-70 now, possibly more. Not even tried counting them When I have my new tank established, new babies will probably be left inside that tank. Its going to be very heavily planted!!