Okay.. This actually has some even more serious implications than we're covering. If that is Cadance and Shining Armor's wee lil baby, then season 3 has to take place some time after season 2. The reasoning for this being that newborns look different as demonstrated by the cakes children here:
This is most likely not the case, as that would require an aging of the Cake's children and the CMC, along with probably Spike, which we know hasn't happened.. So my best guess is that there is a Crystal pony princess of some kind and this is her daughter.
Just my opinion, if anyone disagrees please tell me why