> Children do deserve more happiness, because they're more vulnerable
That does not sound like a utilitarian argument. Are you saying that more happiness should be allocated to children and that vulnerability entitles one to more happiness? Conversely, does that mean non-vulnerable people require less happiness?
> Still, a child's logical utility demand is not _that_ much more to call them a monster, it's within limits.
Well, logical demand is not the concern here but irrational "think of the children" thought-terminating cliches used to justify policy changes at the expense of others. The children are held up as purported utility monsters. I.e. they are rhetorically treated as if they were true utility monsters.
Children do deserve more happiness, because they're more vulnerable and have a longer lifespan, investing in children makes sense for humanity, additionally a parent's well-being is often connected to their childrens' well being. Still, a child's logical utility demand is not _that_ much more to call them a monster, it's within limits.
Utilitarianism's problem is that it's hard to quantify, the logical basis seems pretty solid to me.