Let me begin by saying - one reason to hire a professional is that your kids are too used to you. Trust me, thats a good thing when it comes to most things in life, but when it comes to a photo shoot, they totally do not want to look at you! I know this first hand, because when I do other kids 18 month photos they find me interesting (or odd...but for the sake of my feelings we'll go with interesting) and this helps me to get them to look in my direction for images because I might make a sound they don't hear a lot, or make a face that is new to them.
Case in point - taking my daughters 18 month portraits. She would rather oooh and aahh over her grandparents gardens, than look directly at me. Problem of a photographer's kid I guess, but getting her to look at the camera was as about as difficult as trying to trim our cats nails. It reminded me that I am so lucky to get to work with so many amazing kiddos; and that they, at least, find me interesting!