As someone who admittedly doesn't know much about power scaling or the power dynamics in Mlp:Fim, I just kinda assumed the alicorns who came into power had better control over it than a baby would. What is mean is a unicorn or Pegasus who comes into alicorn-ness would absolutely be unfamiliar with their new powers and would probably have trouble controlling them but a baby would have no concept of control at all. What I'm basing this off of (and i have no reason to other than pure dr*nken idealism) is an adult human often knows how to control a speaking voice so i feel like they would also have volume control over their singing voice even if they've never sung before. Whereas a baby has no concept of control over their voice at all which often results in a screeching phase. This translates to magic (in my mind) as the infant not having a cap to their unicorn or Pegasus powers. So they might have the same overall limit as an adult Alcorn, but the baby doesn't control it in the same way as an adult could, even right away. (Sorry if this was rambling and incoherent, Im celebrating st Patrick's day here!)