It does not seem very scientific, because a) you are not referring to your evidence (at least in this sentence) b) what is "harm"? A more suitable phrase might be (for example) "a clear link has been shown between watching television and a reduction in liver function in children", And then mentioning the evidence or referencing the studies.

No, it's not too informal, but you might also use 'undoubtedly' or 'unquestionably', which are maybe more suitable for essay-writing.