Peacocks, just like every other bird or creature with a gizzard, need grit. The gizzard is more like a mechanical stomach that helps grind food particles to help birds digest them quickly.
There are different types of grit that you can choose from to give your peacocks
Remember that you should only feed your peacocks store-bought grit unless you are very sure of what you are doing or are a pro at raising peacocks.
As for baby peacocks or peachicks, they get as much grit as they need from their starter feed. You will mostly feed your baby peacocks with starter feed, so you do not need to give them any extra grit.
Just like other birds, peacocks need grit to help them digest their food. If you are feeding your peacocks formulated feed, you do not need to give them grit.