I really think that for a child who's working with a tutor, it's going to be easier to slow down than speed up -- if enrolled at a school I might err the other way,
Let's say you do BC. At Christmas, you're halfway through BC and ... it's not going so hot. You slow down, do loads of review, say "Well, we'll do AB instead" and finish AB.
Now let's say you do AB. At Christmas, ... she's bored, you're only halfway through AB -- speeding up to cover BC will be far more challenging.