Hi! and Welcome!!
We have been to Disney Hilton Head lots over these past 10 years and LOVE it. It is one of our home resorts thru DVC. Regarding activities for little ones. The resort sponsors activities for kids throughout the day. They are usually geared for preschool and above and some do cost extra money. For the 12 year old there is even a movie and pizza night you can enroll them in.
As for the little one I highly recommend checking out this interactive children's museum called 'The Sandbox': thesandbox.org.
My little guy (3 at the time) LOVED it. Also, at there is a kiddie pool at the resort and an area at the beach house for little ones to interact with squirting beach balls.
As for dinining reservations we usually go off season so reservations haven't been an issue for us, but there are lots of restaurant guides available and searching ahead of time via internet helps out as well. NOTE: the ONLY downfall for this resort is NOT having an onsite restaurant, but you are only a 5 minute drive away from the nearest grocery store (piggly wiggly). Enjoy!