The "baby" bettas there are pretty much the runts of the spawn.. they will grow with the right environment and food, but they have been known to be weaker and pass away a lot easier than normal.
For them I would recommend keeping their tank temp at a steady 80-82F.. the warmth will help promote growth.
Food I would say start them out on New Life Spectrum Growth formula - it's much healthier than the betta formula and a good size for them.. I usually feed my juveniles NLS at least once a day (mixed in with other foods).. but feed them the NLS at least twice a day minimum.. three times a day is more ideal for them as they are stunted and young.
As for clean water.. don't go overboard, if they are kept in too clean of water they will never grow a strong enough immune system. Their water change schedule will depend on the size of their tank(s), whether they are filtered and cycled, etc. So no one can give you the right water change information without knowing their tank sizes, plants, filters, etc. They aren't fry, they don't need daily water changes at that point..