I've also heard if you get a couple guppies (I would suggest females, brown if you can) that if they start eating the frozen food, the fry will too as they are very competitive for food. I'm not sure if it will work, as I'm really suggesting stuff I never tried.. but seems to make sense.
Just be consistent, and they will learn. Slowly, only ever so slowly, I would cut back on grindals too. I'm sure if they are stuffed on the nice food, they wouldn't want to eat the frozen (other than being pigs and will just keep eating).
Hmm... that's all I can think of now. Good luck.
Oh, how convient Paul popped up when you said "black cambodian". :P