Not sure what the best is, but my opinion is that the best idea for the little guys is a paper (like paper towel) or "cage carpet" substrate and then a humid hide (plastic container with damp moss or coconut husk product.)

I like the coconut products - Eco-Earth, etc - for baby snakes. Easy to keep the substrate and snakes hydrated. I've fed babies on it. No problems so far.
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Mark