Can you use witch hazel on a baby?

You can, and likely with complete safety. I would recommend diluting it to 3:1 with water (3 parts water: 1 part witch hazel). However, the benefits of Witch Hazel are pretty limited and are in question, so no promises on if it will help whatever malady you're treating. The primary active ingredient is tannin, which acts in this case as a mild astringent. If a rash develops, welts etc. immediately discontinue usage. A note on all herbal and homeopathic remedies: Unlike conventional pharmaceuticals, these are often made from living organisms, and as such the contents and dosage is far more likely to vary.