If the goal is the happiness of all humans, then we should bring our numbers down in the most humane way possible: have less kids or no kids, and adopt. While I understand that it's a very difficult process to adopt, it is much more humane than increasing the world's population and leaving those other kids to not find homes. One thing I agree on is that bringing population down by killing people who have already been born is not the answer, nor is trying to decrease the birth rate through laws, as in China, where the result is a high rate of female infanticide. It would have to be something that most of society would have to agree to do voluntarily. This would require a change in the culture, which is never an easy task, as cognitive bias makes the human mind extraordinarily difficult to change.
An adopted child will not love you less than a biological one, and hopefully you will not love them less. They may not have your DNA, but that has very little to do with how someone will turn out, compared to upbringing.