Utilitarianism based on happiness would be just one branch, maybe even “hedonism”. I’ve found that the evaluation of happiness involves an increase in balance on a person’s values, or avoiding a decrease and satisfaction of needs. Someone who has dead has no values and no needs and can not experience happiness. They don’t enter into the equation, (but you could argue that the process of murdering them creates an abrupt amount of unhappiness until they finally succumb, but so do a lot of relatively happy activities like childbirth and working a job you hate for monetary reward. Unless you value a state of non-existence, the idea of no longer existing doesn’t make you happy, whether or not you might be incapable of feeling unhappy later.
I think utilitarianism would look more at the utility of the person being dead on survivors. The friends and family that would lose the benefit of a relationship with that person being alive. You could weigh that against the benefit people would have by the loss of that person, (maybe inheritance, insurance payments, or simply having more resources available). Perhaps someone extremely wealthy would be more valuable to society dead than alive, if their wealth was distributed around, (that’s a variation on the killing of a healthy individual to harvest organs). But we could say we should avoid these acts because of the utility of avoiding that rule in case it could later be applied to us personally.
Having a child shares a lot of the same utility. It has been shown that having a child is expensive and the happiness of the parents rarely increases, but the relationship with the child is something of value to the parents, as well as meaning to their own lives, which means a general increase in utility. However, that has to be weighed with whether the child reduces the utility of society by using too many resources and significantly reduces happiness in society in general.