I don't believe that we zoophiles are in favor of the person's actions if the person had sex with baby animals.
I was under the impression that one of the reasons people argued in favor of animal-human sex is because animals aren't cognitively and socially predisposed to obey the authority of adult humans as human children are. If that's true, then wouldn't the same apply to animal children?
I'm not certain that every child who has sex is necessarily significantly harmed by it. I need to research the topic more myself in order to know this for sure.
There are certainly children who have been sexually abused by an adult who did not suffer trauma or harm as a direct result of that incident. I, myself, am one of them. In my case, it was because I had no idea what sex was and had not internalized any connotations about it. The harm came after I learned what sexual abuse was, that what had happened to me could be considered abuse, and internalized a lot of societal messages about sex.
Something you might want to look into is the the Rind et. al controversy. More information on it here. There were two peer-reviewed meta-analyses, one done in 1997 and one done in 1998 and the authors of the analysis conclusion came to the conclusion that
harm caused by child sexual abuse was not necessarily intense or pervasive,[3] that the prevailing construct of CSA was not scientifically valid, as it failed empirical verification, and that the psychological damage caused by the abusive encounters depends on other factors such as the degree of coercion or force involved.
which caused a moral uproar and Congress to officially condemn the study. The lead author, Bruce Rind, enforced the idea that the wrongfulness and harmfulness of child sexual acts are not intrinsically linked:
the findings of the current review do not imply that moral or legal definitions of or views on behaviors currently classified as CSA should be abandoned or even altered. The current findings are relevant to moral and legal positions only to the extent that these positions are based on the presumption of psychological harm.
—Rind et al. (1998) p. 47