IMO you should start doing water cahnges much earlier (4 days old) and 50%water changes, I have another tank full of treated, heated water I use for water changes... I have fry who are 3 months old, and the size of 6 week old fries. :( From not doing enough water changes.
And a few fry will eat frozen food, but for the most part, they will not eat it. So its good you didn't buy any :) I wasted money buying my bettas some :( luckily my guppy fry gobbled it up :D
I rely on microworm cultures, Atisons Starter, and BBS for raising my fry. Be careful, too much BBS results in SBD and will kill most of your fry :( After the fry are about 6 weeks, I start feeding them powder food, like Atisons betta Starter - but I've heard First bites works well too, though, i've never tried it..
and last but not least, I HIGHLY suggest you get a sponge filter(about 3 dollars on ebay), and not use a regular filter (either hanging, corner, quiet flow that hangs inside the tank) A small mini sponge filter, will help, as betta fry do not breathe air until 7 weeks old. So you should have some air going in there. Have a gang valve to restrict the flow of air to about a bubble per second. I never turned mine off, ever.
Also when your microworm culture gets to your house, be sure to have oatmeal :D I didn't have any and ended up using potato bread, but oatmeal works 30x better. . .
You need to be feeding fry their first meal of microworms when they are free swimming and you remove the father. Not all will need to eat it, as some may not be free swimming yet (but dont worry), the ones that absorbed their egg sack the fastest will need food :) and the worms will swim around for a few hours. But if you find you have a lot of dead worms on the bottom of the tank, you're feeding too much.