Maybe it has nothing to do with spirits or angels but might be more based on down to Earth, reasons?
For example: Perhaps a baby that looks into mirrors will grow up being vain and could G-d forbid wind up being like some who always have to worry about their looks, and who can’t pass by a mirror without admiring themselves?
So if as babies, they are taught not to look at mirors, perhaps they might be less likely to get that characteristic of being so vain, and self absorbed, later on in life?