Treating anything on fry can be tricky especially in the really young Betta fry.
Get the water temp up slowly to 84-86F but watch the fry for distress, add 1tsp/gal aquarium salt, add small amount dissolved in tank water over an hour, watch for distress
Start making daily water changes and get the bottom of the tank really good to try and manually remove the parasite
If only a few of the fry have ich on them I would remove that fry to a separate container and treat too.
How old are the fry and is the father still with them, sometimes the fry will not make it depending on their age.
I have never used any type of medication with any of my fry for parasites and I have had fairly good success in treating fry with temp, WC and salt with 50% surviving.