Feed them medicated chick starter around 18% protein. Don't get too high of a protein % because it might cause leg issues. I have no experience with other birds hatching and raising peachicks so I can't help you with that, but I would keep an eye on them. Seeing as turkeys and peafowl are around the same size and turkey chicks look kinda like a peachick I don't see it being an issue, but I only raise peafowl here so I can't say.
When I have had my peahens hatch and raise the peachicks they normally do a great job so I don't see why some of yours died...Do you have chickens? They could have gotten black head. Also, if you don't have small fencing they could have wandered outside of the fence and gotten eaten by a predator. That is what started happening to my peachicks and I think the hawks learned that Spring and Summer time means peachicks because my dad said they were circling over my aviary like they were looking for something.
Hopefully the medicated chick starter will keep them healthy. They really like to eat wet medicated chick starter. Sometimes my peachicks would almost refuse to eat it dry.