Things I didn't know before having children include, but are not limited to, the following:
-that while Addy is eating, trying to keep her and the dog away from one another is as futile as the Capulets and Montagues trying to keep the infamous star-crossed lovers apart.
-that baby nails are the sharpest weapons known to man.
-that breastfeeding is NOT always painless and effortlessly natural.
-that when one has a child in tow, complete strangers seem to feel that the boundaries of respect and privacy are removed and they are free to, say, yell sharply from three yards away, "That baby needs a hat!" And that even responding politely with, "Well, he actually had one on and was crying and trying to pull it off, so..." does not shut said strangers up. Instead it apparently invites them to respond nastily with, "Well, he's not the boss, you are!" over their shoulder as they walk away, leaving you gaping and fuming and thinking up wonderful retorts just a few minutes too late.
-that a rather long, lean two year old is actually easier to carry for long periods of time than a short but remarkably solid bundle of two-month-old.
-that introducing vitamins to a child only gives you one more thing to say "No" to when your kid starts asking for vitamins five times a day. Try explaining to a frantic toddler that ONE vitamin is a good thing, but LOTS of vitamins will make you sick.
-that potty training will involve many episodes of spotting a child about to go, rushing them to the potty where they twiddle their thumbs for ten minutes, then putting on a fresh diaper only to have it promptly soiled about a minute and a half later.
This is just off the top of my head. Feel free to educate me further and contribute to the list!