Congratulations! I have a 3 year old son and he was my first and only son. I asked my ob/gyne and some other people that I liked and trusted who their children's pediatrician was. My husband and I interviewed pediatricians (which we had to pay for)and ended up selecting our ob/gyne's suggestion. It's where he takes his little ones to as well.
In the interviewing process, my husband and I were interviewed about our health histories along with family health issues. We asked them about how they deal with sick children (is there a special time when kids that are sick can come into the office and not disturb healthy ones), hospital affiliations (we had a preference for Northwestern)and simply whether we liked the person or not.
Best of luck to you and your expanding family!