It is normal for livebearers to eat their own young,,it is possible that they did kill the dad to but more likely he died or was dieing and they scavanged the dead/dieing body....get lots of plants . ..( live or plastic) and create a few very densly planted areas and a lot of the babies will manage to make it. I have guppies and that is what I do . I am not sure if swords are more aggressive about eating their own young than gups or not.good luck.....oh and also make sure that the intake tube of your filter ( if you have a
HOB) is covered or you will have the babies bein g sucked into the filter and they will be killed that way ...I use a sponge to slide up over the intake stem and it works real well...also works as a prefilter and as a bio area for bacteria to grow.