A few things that I wish I knew about before having kids:
1- save money by buying baby, infant, and toddler items used...craigslist, thrift stores, handmedowns from older cousins, etc. Put that saved money away for their college education! It never occured to me to buy stuff used. We got a lot of nice handmedowns for my first child from an older cousin (thank goodness!). Other than that, we bought everything new. We could have saved thousands of dollars if I just did a little research and bought things used.
2- that the disrespectful behavior and attitudes start way earlier than puberty/teenage years. Try age 3! I expected the tantrums and meltdowns, but not the sassy attitude at such a young age.
3- that "Mommy Guilt/Worry" can be so overwhelming.
4- that it's really okay to not be "Super Mom" and to be selfish sometimes and put yourself at the top of the list. I still feel bad about spending money on myself by getting massages every couple of months and taking Saturday afternoons to be by myself, but I know I need that time away from the duties of children, husband, and household. I'm a better Mom and wife because of it. I just wish I knew that during the first 2 years with my first child.