Kourtney,
You have come to the right place! The beauty of traveling to the Walt Disney World Resort is that it is truly a vacation spot where there is fun around every corner for every age group. I have 3 kids that are pretty spread out in age, so we have definitely traveled when the kids were at different stages of their lives.
Kids of all ages can ride most all of the rides throughout the parks. Unless the ride has a height restriction, kids of any age can ride the attraction with an adult with them. Meeting characters is often a fun experience, too, for both ages. Keep in mind, some tiny tots absolutely LOVE the characters, whereas others might be more hesitant to approach them. So, a handy tip is to familiarize kids with the characters before your trip and don't push them, it may take a day or two for them to get comfortable with the characters. There may be a time where the 5 year old wants to do something while the little one naps, so at that time, focus on an activity or attraction that might would appeal only to the 5 year old!
There are a few hidden gems in the parks that are perfect for that age group such as the
Affection Section in Rafiki's Planet Watch in Animal Kingdom where the children can bond with live animals in a petting zoo type of area. The Country Bear Jamboree in Magic Kingdom is an adorable attraction that my younger kids have always loved (it's one of my favorites, too). There are play areas in some of the parks like the Honey, I Shrunk the Kids Movie Set Adventure in Hollywood Studios and the Boneyard in Animal Kingdom.
May your vacation be filled with lots of memories and cherished moments! Let me know if I can answer any more of your questions.
~Allison L