Betta fry are difficult to raise. One of the toughest things is, well what do they eat. Do not try using powders or flakes, it will just totally foul up the tank. The best thing I have used is microworms. These are tiny little wiggly worms that you can grow yourself and never run out of them. I purchased my first microworm culture from a site called aquabid. Make sure you get this culture before your betta fry are born. Like probably a week before. When the fry are swimming freely, and ready to eat then its time to feed them these microworms.
What I use is a mashed potato mixture to a consistency of like peanut butter. I just used potato flakes and water and put them in a small clear bowl with a lid with small wholes in the top. Place a tablespoon of the culture you got from aquabid in the center of the bowl on top of the mashed potatoes. After about a week, you will see them crawling up the sides of the bowl. Sounds kinda icky, but its not too bad. Then you can take a Q tip and run around the sides of the bowl pulling off the microworms. Then just empty them in the tank off of the Q tip. You will be able to see them wiggle and see the baby betta frys eat them. Do this twice a day.
Its good to have 2 to 3 cultures of microworms growing at all times. All you have to do now is repeat the instructions above and you will never run out of food for your betta fry. The culture usually last about a month or so, so always make new mashed potatoes and take a table spoon from the one you grew and start another one.