Centaur theory

This depends on a few things and with some considerations, baby centaurs would probably be best suited to nurse from the human teats, rather than the equine teats.
The first thing to consider is whether the offspring is full Centaur or not. Will your story contain a race of centaurs, complete with families, culture, cities, etc.; or will the story contain a small selection of centaurs or possibly just one, as a magical/mythical being?
If the centaur(s) in your story are mythical, then there is a high likelihood that any offspring of the Centaur's is half of something else too (i.e. human, horse, god, faun, minotaur, gorgon, etc.), in which case, the mythical nature of the scenario can allow you to forgo answering the question entirely via a number of interesting ways, the easiest being the spawn is born mature.
If the centaurs are a race and have an obvious biological component, then the offspring is likely not a mix breed and you must indeed answer this question.
Pros and cons of the human teats: A centaur baby will have a human head and a human mouth. An infant mouth and a mother's teat are very much like a lock and key in most mammals. The teat is correctly sized and adapts temporarily to each infant's nuanced differences. However, being half horse, we can consider that the infant supports itself much, much sooner than an human infant, so the mother would not be carrying around a baby for months, but rather weeks or even days. The human teat would be too far to reach easily.
Pros and cons of the equine teats: An equine teat is far too big for a human mouth, especially an infant's. Also, an equine teat would deliver far more flow than an infant would want. But, it is conveniently positioned; the infant could reach it quickly and easily without much burden to the mother.
Some have noted nutritional differences, but I think that's an odd distinction if we already assume these centaurs are a biologically natural species. If they are, then the milk is biologically suited for centaurs regardless of which teat it comes from.
So with this in mind, the human teat makes the most sense, but considering centaurs are neither human nor horse, there's no reason to assume that they behave or develop similarly. What makes the most sense to me is that Infant centaurs are weaned relatively quickly from the human teat. I would say about a week. They also develop rapidly, growing the human equivalent of four or five years in only a few months. The equine teats are never used and you could even say that they do not exist.