Certainly sounds like a fungus / external bacterial infection. Any chance of a photo to make sure ?
Be very careful how you treat the betta - they are very sensitive to most meds and will react badly to most full strength medications. Ask the folk over in the betta section for advice.
also get yourself some Melafix... its expensive, but damn good !! its made from tea tree and is a naturally medication. smells lovely too !
Be very careful with melafix and your betta - you should only dose at half strength. Ideally you should use Bettafix instead - specifically made for bettas.
Melafix should be fine on the other affected adult fish.
As for the fry - if you treat the tank, they will almost certainly die - sorry to say

If they are not affected, you can try removing the affected fish and take your chances with the fry in the main tank.
These kinds of infections / disease is more often a problem with water quality.
Make 100% sure that you are indeed treating for the correct disease.
Do you have any idea what your ammonia, nitrite and nitate readings are ?
Finally - never use two medications at the same time.