Here's why:
In a murder case, greater scrutiny is given to evidence(we hope) and in the vast majority of cases, there is a body as proof that a murder occurred. Also, there's usually a motive involved that makes some sense, and sometimes a murder weapon. When one weights the checks in place, it's a pretty slim chance that an innocent man will be executed.
In contrast, children have been proven to be malleable in their testimony(see the McMartin case and related hysteria in the 80s) and many innocent people can be ruined for life. It's one thing to smear a reputation, another to execute an innocent man with no evidence but the word of a frightened child, out of their depth. Even several children's testimony put together isn't a guarantee of truthfulness, as, once again, the McMartin case in CA proved.
I agree on extra-long jail sentences, though. I would also point out that some laws regarding "sex offenses" do not treat a flasher, or exhibitionist or boy who slept with a girl two years younger any different than a man that raped a young child. Perhaps if you limit it to child RAPE cases(not just molestation, which is difficult to prove) then you'll sell me on that.