The rebuttal is offered that we can't be obliged to do that since our ancestors didn't, and they got on okay.

Sounds a reasonable, if a bit overly careful, strategy if one is seeking to maximise virtue (in the sense of desirable traits) rather than trying to avoid harm. We currently seem to believe many of the things that are virtuous are brought about by not doing harm or being benevolent to the child, there obviously exist sets of virtues for which this is not true. Also, we may be wrong.

But this rebuttal is a fully general counterargument against any moral change, including changes that seem obviously correct; indeed, it would have been a counterargument to the abolition of all sorts of things that we today consider atrocities

History is not homogeneous when it comes to norms. All sorts of practices where adapted then later abandoned. There are plenty of things people practice that made them turn out great or ok that we or our great-grandparents didn't practice.The argument can thus pretty consistently be used to change the status quo and create something like "moral progress" or rather a process of "moral change". You can't really mine out history, since we may have faulty ideas about what was done and all processes of upbringing will be imperfect. Thus we'll be trying "new stuff" without knowing it, if it becomes known that we did, congratulations we have just expanded the space of possible approaches on which we have empirical results.

I don't think this works as a general counterargument to all moral progress. I think it just maps to a different systematised morality than the one you or I might come up with.