Hi Jessica!
My husband and I love doing runDisney races. It's a great and healthy activity for us to share:) Plus, it combines our love for running and our love for all things Disney!
There are actually seven different race weekends between Disney World and Disneyland. The Princess Half Marathon is the one you are referring to, as it is held at the end of February/first of March. Generally, it is one of the last two weekends in February, but has occasionally been the first weekend of March. You can find the race calendar on the
runDisney website. Usually, they post the race dates about 8-11 months in advance.
There is not a separate area for kids to watch the race. When my husband and I both run, and need childcare, we use Kids Nite Out. (www.kidsniteout.com) It's a great service with highly-trained and screened childcare providers. There are some Kids Races and Family Fun 5K's during race weekends if your family would enjoy a race together!
I hope this helps! Please let me know if you have any other questions about planning your trip!