Keep in mind that drinking tea and tea tree oil are two completely different plants. Tea tree oil comes from the Melaleuca alternifolia. I don't know that the camellia sinensis is going to harm our hedgehogs (I honestly haven't really read much about green tea extract in shampoos and its benefits/dangers). But I always try to err on the side of caution. If I'm not certain I don't do it. In this case, I would stick with things I know work fine (Aveeno body wash for mine).
How often do I bathe hedgehogs? Only when they need it. I cannot tell you off hand the last time Rose had a bath. Its been a long time. She is an extremely clean hedgehog and occasionally just needs a wet wash cloth to get a spot cleaned. Typically if I do have to give a bath, its with just water. If for some reason I deem it necessary to use soap, we use Aveeno Body Wash.