Hi Jessica,
I'm so excited for your first Disney trip. It's going to be a wonderful experience for your family.
Entrance to all Disney rides is based on height. Both of your children may go on any ride for which there is no height requirement. This includes dozens and dozens of attractions including, Winnie the Pooh, the teacups, It's a Small World, Buzz Lightyear Space Ranger Spin, Jungle Cruise, Country Bear Jamboree, the carousel, and much, much more. You can check out the
height requirements on the attraction pages on the Walt Disney World website. Additionally, there are character greetings, parades, fireworks, and other activities that the entire family can enjoy together.
Unless your 4-year-old is exceptionally tall, there will likely be very few rides that she can go on that your son can not. There are no rides that either of your children can go on alone. The attractions at Disney are designed so that families share them together.
Don't worry at all about finding enough to do. There is plenty to entertain guests of ALL ages.
Have a great time!