Can I use diatomaceous earth? It's supposed to get rid of pests.
DE is a respiratory irritant. Chickens have VERY delicate respiratory systems.
If your coop is infested so that your baby chicks have parasites it's much better to use a truly effective insecticide. Here's a useful article: https://www.backyardchickens.com/ar...-fleas-dealing-with-external-parasites.74599/
If your coop is not infested and your baby chicks don't have parasites then there's no reason to treat them for anything.
A dustbath of plain dirt is all they need to keep their feathers in good order.