I am not sure about the relevance of your point. I never averred that the parents or the states of affairs aren't pertinent. But it's true that the decision is about a person. Moving on.
In isolation, the damage is obviously unethical. However, when the act can lead to greater happiness for a person, it can be justifiable to do so. I also don't think that one is acting for "someone else" when nobody exists at the time of the act, but I shall ignore that for the sake of the argument.
The harm might be unnecessary, but the happiness isn't. If it's necessary to prevent harms even though preventing them doesn't lead to a good for someone in an alternative state of affairs (in the form of fulfillment or relief), I think that it's also problematic to never create any joy.
If it leads to greater happiness, it is ethical, in my view. It's definitely about the ethical act committed by the parents of creating a good.
Benefits are also ethically relevant, particularly when one is not in an already satisfied state of affairs that they would be mostly happy with as long as serious harms are avoided.
In one case, there won't be any positive. In the other state of affairs, there would a joyous experience for an actual person.
It is indeed bad because it's absence does lead to harm for an actual person (such as the lack of health leading to suffering). One could claim that some goods are supererogatory as far as existing people are concerned because they don't need constant interference for living fairly happy lives as long as they can avoid serious harms. But this isn't applicable to those who don't exist.
If the lack of happiness isn't bad because nobody is deprived of it, then neither is the lack of damage good, since the absence of the negatives does not provide someone with relief/fulfillment resulting from a satisfaction of their interests.
If we are indebted to "them, prevent damages for "them" even though "they" did not express any interest in it and neither does the prevention ever lead to an actually better state of affairs for a person by giving them some sort of benefit/relief, then we definitely have to take the joys into account.
In the end, you consider the damage to be unnecessary (which I also do, but in isolation of other factors) but the creation of joys not necessary. But I disagree with that because my intuitions tell me that they are quite relevant.
Anyway, this has been repeated multiple times. I hope that you have a wonderful day!