I read that you can make fry food out of boiled egg yolk, and smear that on the side of the tank, which the fry will eat off of. It gets messy fast, so be prepared for more frequent PWCs. Of course, by Darwin's law, you'll raise discus that are prone to eat their fry. Give the parents a few more tries, they mey eventually learn to not eat the fry.
Try to keep the area around the tank as stress and traffic free as possible. Stress will generally lead to baby munching. And discus don't like heavy foot traffic around their tanks.