You tell them not to eat it! It is as simple as that. If they try to eat it you give them a time out or a paddle on their butts. It will work, trust me on this.
Or I have no idea how to keep little ones out of big ones feed. I just looked up the stats on feather fixer and I see it is 18% protein. Protein being the key I think during molt, why not use a game bird feed. I am feeding everything a 22% game bird finisher right now because of the molt and it does not hurt the young birds. Oh, I feed a mash too. My birds look at pellets or crumble like my kids use to look at spinach or liver, there is no way they will eat it.
Telling them not to eat it didn't work.
So it shouldn't hurt the little ones just because of the higher protein ... wonder if FF has the 22 % game bird, I'll check next time
I'm there.
THX