Let's cut to the chase here.
Let's suppose I bring forth an 8-year old girl who meets the standard of competence required to give "informed consent." (There are undoubtedly some who might qualify.)
Are you telling me that it would be OK for me to begin a sexual relationship with her?
Probably not -- in which case your reasoning would have to involve something other than informed consent.
But this simply points out the difficulty of reasoning with people like pedophiles. Any other objections you can make revolve around some version of "doing no harm." The problem is that for to each plausible objection you can make, somebody can make a plausible counter-argument. We see them being made -- it causes no physical or mental harm, enhances sexual performance, etc.
Eventually you're either going to have to accept pedophilia, or you're going to have to say "it's wrong because it's wrong."