It might be nonsense, but my Mum was suggesting that feeding live mini mealworms to the male betta might have gave him a liking to live food; thus consuming the newly hatched fry. I rejected the thought at first, but come to think of it, it might be true. I didn't feed any live food to my first breeding betta pair and all went too well on the first try.
Well there is no harm in conditioning the pair with good quality food and that's what I'm going to do. No live food for the male betta prior to breeding.