I like anything that can clearly be differentiated from feed. Sand is free for me so that is what I use. It works well since it is a poult's instinctive behavior to peck at things on the ground. I sprinkle the starter on the sand bedding and my poults usually start eating within minutes of...
I would not put poults or keets on ot for at least the first two weeks and definitely not without providing them with grit.
I live on a sand dune so my choice for bedding is sand that I dig up.