The fry will not feed from a BBS "feeder". You have to hatch out BBS in a cone and then harvest the BBS and squirt some onto the parents sides...you will see the fry start to dart about after the BBS and ignore their parents. If you want to wean them off the parents quickly, feed them the BBS in the above manner 3-4 x a day until all the fry go for the BBS. BUT, when you start feeding like this, you have to keep the tank super clean with large daily water changes to prevent any bacterial infections or deformed fry later on. Ideally, the fry should be pulled right now and placed into their own 20-30 gallon tank and fed heavily with BBS and large daily water changes. But since the fry arent weaned from the parents, it'll take a few more days.
Oh, BTW, to measure yourself against the standard for growth in discus fry...they should grow 1" per month. So at 4 weeks of age, they should be an inch long...by month 3 they should be ready for sale at 2.5-3". This is very hard to achieve, but it is very possible if time is takin to do things right. They will continue to grow at this rate until they reach a size of about 4", then their growth slows to about 1/2" per month or less, depending on color type.
-Ryan