Melanie,
Nope, no ticket needed for your 9-month old! Children under 3 do not have to buy tickets, and can eat from their parent's plate ate meals for free as well.
As for your 4-year old princess, there are a lot of things to do. Here are a few of my favorites;
Princess Autographs: Find out where the princesses are for meet-and-greets in the parks during each day's visit. The princesses (and other Disney characters) will also sign an autograph book, if you want, and this is a nice way to capture the memories. Other than buying an autograph book (or bringing your own) there is no cost. We like to glue in pictures of our daughters with the princesses when we get home to make the memory
Dine with the Princesses: Sometimes these meals can be pricey, but the character meals are a great way for her to meet as many as 5 or 6 princesses in a short period of time.
Bibbidi Bobbidi Boutique: This child-focused salon performs princess themed makeovers that have become very popular.
After that, just focus on many of the princess themed attractions, shows, and events. They really can elevate the experience. Some of my favorites include Enchanted Tales with Belle (Magic Kingdom), Under the Sea (Magic Kingdom) and the Belle stage show (Hollywood Studios). Write me back if you want some further selections. Let me know her favorite princesses and maybe I can come up with other ideas!
Have a magical vacation!