Amazing, isn't it?
I really don't think these painters lacked the skill to paint a baby so there must be some reason that holy children (I'm including angles and such) are presented with adult features.
And isn't it interesting that Mary is presented as royalty in so many of these paintings?
I've spent many many hours of my life in art history classes and cannot recall such editing of images. You would see details of images but not something like a hand edited out.
Its a matter of controversy in the art history world, which is partly why you won't have seen it. Also, it would be more likely to occur in journals/papers than in your classes.
I think it is a pre-Renaissance/post-Renaissance thing.
pre-Renaissance everything was stylised with no perspective, Mary and Jesus were largest and sitting on a throne, surrounded by angels or saints who were a bit smaller and at the bottom of the painting was usually a row of people including the disciples and the patron who had paid for it, smaller again. True relative sizes were irrelevant, the importance of the character dictated the size.
Realism wasn't an issue so the reasons of divinity and therefore a greater intelligence to be shown than a normal baby could be an issue?
The later you get into the Renaissance the more 'real' the babies become - but at the same time so do the other people, they become real recognisable characters and not simpering stylised figures. Crucified saints not longer gaze mournfully up to heaven but actually start to suffer!