Hm. Ok. There may still be something stressing the birds, so I would really wrack your brain about what could be near the cage that might stress them. They also badly need a diet change, it could be that the stress of feeling ill from the diet is making them cranky enough to kill chicks, or their bodies could be telling them that the conditions are not right to raise a family because of the diet. I suggest switching to a fresh food diet, or at least pellets, but the switch to fresh is easier and I believe healthier. The easiest and cheapest way to overhaul their nutrition is to simply sprout the seed mix you are already giving them. It is amazing the wonders this can work in a bird.
One thing that most people wouldn't think to check for, but if you have a mouse or roach problem, it could be that stressing them out. In the end, it could be that you will not figure out why they are doing it, in which case I hope you will stop breeding them. In the mean time we will try to get it figured out.