So that child abuse is objectively wrong, independent of any opinion. No matter if lots of people believe it today, as you point out. Or if no one believed it, and everyone was raping, killing, and eating two year olds. Yes?
If so, what is the source of this objective morality?
(It is confusing because you seem to be saying different things that seem to be inconsistent. The fact that slavery was less wrong when it was accepted than it is wrong today, now that opinions have changed. Is that one of the "wrongs" that is not 'actually' wrong, just subjectively wrong? It must be if it changed with opinion.
If everyone was raping, killing, and eating toddlers, would that too be less wrong then than it is now with nearly everyone's agreement? And would it be wrong in a remote jungle tribe that raped, killed and ate toddlers?)
If so, why?
Yes. I believe you said that you believe that there is an objective truth. A really real, independent of anyone's perception or opinion. If that is the case, how can you know that you know it? What makes you believe that you can correctly know and perceive it?