Here are some reasons for why having children may be altruistic (With the caveat that I haven’t engaged deeply on this subject):

1. It is better for that child to have existed than not. Contra the VHEM, I think there is pretty good empirical (and subjective) evidence that it is better to have lived than not to have lived, and that in the developed economies most lives are significantly net positive. Therefore having a child will create more happiness than sadness (from the perspective of that child, as well as to you probably and their future loved ones.)
2. From a longtermist perspective, as long as principle 1) holds and as long as your children have children (which is statistically likely) you will help continue the chain of civilisation, with future net positive lives which stand in relation to you as you do to your ancestors.

3. If you are someone with deeply held ethical beliefs and a wider than average moral circle (which feels very likely given the context), then having kids will likely be moral moral than for the average person. This is because your kids will likely inherit your ethical worldview (to some extent) and they may choose to have positive impact through their actions (e.g donations/career) or by promoting those values to others (through conversation, political activism etc). One way to think of this is: what would happen if all good people didnt reproduce, and only people who didn’t care about morality had kids? I would guess that the short-term benefits of good people having more resources would be swamped over a few generations by the negative ethics of a corrupted culture.
    

   there are significant counter arguments to consider as well (e.g the meat eater problem, the  opportunity cost of having very expensive children reducing capacity to donate) but I think the above reasoning shows why having children isnt firmly on the buying a sports car side of things in my mind.