I'm not sure about this. It's most likely that anything your kid does in life will get done by someone else instead.

True - we might call the expected utility strangers get a wash because of this substitution effect. If we say the expected value most people get from me having a child is nil, it doesn't contribute to the net expected value, but nor does it make it less positive.

There is also some evidence that having children decreases your happiness (though there may be other reasons to have kids).

It sounds as though that data's based on samples of all types of parents, so it may not have much bearing on the subset of parents who (a) have stable (thanks NL!) high living standards, (b) are good at being parents, and (c) wanted their children. (Of course this just means the evidence is weak, not completely irrelevant.)

But even if this is true, it's still not enough for antinatalism. Increasing total utility is not enough justification to create a life.

That's a good point, I know of nothing in utilitarianism that says whose utility I should care about.

The act of creation makes you responsible for the utility of the individual created, and you have a duty not to create an entity you have reason to think may have negative personal utility. (Strict utilitarians will disagree.)

Whether or not someone agrees with this is going to depend on how much they care about risk aversion in addition to expected utility. (Prediction: antinatalists are more risk averse.) I think my personal level of risk aversion is too low for me to agree that I shouldn't make any entity that has a chance of suffering negative personal utility.