For me, I think one might be that children are asexual (I'm not saying this is true but if it were this might be among the least problematic arguments) so there's no possible advantage for them (maybe it could be play but no direct sexual advantage/pleasure) to compensate for risks and although they are still, for whatever reasons, emotionally neutral about it at that age they will become deeply disgusted by it later on in life when they become sexual (or maybe they'll just develop a stronger capacity for disgust. Small children seem, to me, to have a weak disgust response and also little to no body shame or self-consciousness, I could be wrong). The 'informed consent' argument almost makes sense if they don't understand the feelings they will come to have about the act or even those of their partner (I say 'almost' but, to be clear, I don't think it does. Even if regret for this reason is something to consider, it's not the same as someone intentionally deceiving you about a product that you would otherwise not buy when you are capable of making an informed choice about purchasing it. Fraud requires that a person is capable of making an informed choice but their ability to do so has been deterred).
It must ultimately come down to disgust or emotional distress for me. Even with the impressionability/intimidating adult social coercion angle (which doesn't consider personality differences within an age group, adults can exert peer pressure on other adults who might be more approval-seeking or conflict-avoidant and some children are very rebellious, defiant or strong-willed) you can only single out sex if there's a reason to think that it will be especially distressing. I hope everyone would oppose an adult coercing or intimidating a child into playing video games with them but you don't need to stigmatize or discourage adults playing video games with children per se in order to do so, the problem would be with the coercion and not the thing the child is coerced into doing itself.
It also makes sense, on paper, to assume that children who aren't yet capable of ovulation or sperm production are asexual and not 'ready' for sex (I guess in that they would find it disgusting, awkward or distressing in some way; because their bodies are not 'designed,' at this stage in their development, to sexually reproduce) even though most people can probably remember sexual feelings at a young age (say 6/7), babies have been observed masturbating in and out of the womb and not everyone is disgusted by the idea of sexual contact with at least some of the people they're not attracted to either. Even I will assume that libido isn't 'fully developed' until one has a menstrual cycle or is capable of ejaculating, if not with the start of puberty (which 'should' be two years prior to semarche/menarche), but reality seems more complicated (some testosterone and estrogen is present at small amounts from birth, maybe that explains child sexuality).
I do think that we should discourage child-adult sexual contact in practice out of risk aversion (the high likeliness of internalizing societal beliefs about the exploitativeness of child-adult erotic intimacy and later coming to regret at-the-time desired sexual encounters with older people in retrospect for that reason) so I specify 'the best arguments that you still disagree with' (and you can mention why you disagree with them or why you think they're less flawed than standard anti-AMSC on principle arguments are. I'm also open to critiques of my 'in practice/for now at this stage in history' anti-contact stance).
You can even mention anti-AMSC arguments that you agree with.