According to WebMD, herbal teas are just fine to be taken by kids, but it should be only in limited amounts. Although herbal teas have been found to have lots of benefits, it doesn't mean they're completely free from anything that's bad. Some teas like green tea contain caffeine which may cause palpitation and dehydration if taken in huge amounts. Bottomline, just because it's natural and herbal doesn't mean it's safe.
You can check this link to know what herbal teas are safe for your kids
http://www.webmd.com/balance/features/herbs-for-kids-feature