I had fry before even though I didn't know did my piranhas mate..but it happend.
make sure the filter is not too strong so you don't have to worry about those fry being suck into the filter; or you can use some filter cloth to coverthe filter. This way you can maintain filtration and can avoid fry being suck into filter.
in the beginning they will just use up the yolk (the yellow thingy hanging on their belly), then you can start feed them brine shrimp.
when they get big enough you can feed them guppies..just make sure the guppies are free of parasites or diseases.