Is Dried Catnip Harmful To Kittens?

No it's not harmful, it's just less potent.

Almost all cat/kitten toys are impregnated with cat nip.  I would not use cat nip on kittens still nursing so under 6-8 weeks of age.  There are some anecdotal responses that kittens less than 4-6 months do not respond to cat nip--in my experience this varies widely in individual cats.  I would only use it in small amounts to make sure the kitten does not ingest much--large ingestion have the potential to cause vomiting and diarrhea.