I thought the point of a utility monster was simply that they were more efficient at turning resources into utility - I suspect both notions serve to make similar points! I agree that children aren't utility monsters in the sense that you describe, but I think they are at least agents that turn resources into utility more efficiently than adults. I think the claim that such beings could exist is a conclusion that some have claimed is a dodgy consequence to be avoided - which obviously isn't the case if such things already exist.