You could just keep the fry tank clean, I agree with that. I wouldn't try to dose the fry with anything. Just keep anything you put into the infested tank away from the fry tank.
So, the female gave birth and you want to put her into the fry tank? Is it the mother? Is it from the main tank that has the parasite? If it's from the main tank I wouldn't put anything from that tank in with the fry. I'd treat the whole main tank as the others said with the tabs and the food, the betta with the food, and just keep the fry tank clean and see what happens.
Good luck, this is a lot to take on. I hope everyone gets better soon!