Hi Kayla!
Happy early birthday to your big 4 year old! I am also planning my son's 4th birthday at Disney this year. Rainforest Cafe is a great choice! The food is oh so yummy (I love the Chimi Cha Cha's) and the atmosphere is so much fun. There really is no difference in the two locations other than location. If you are already going to be at the Animal Kingdom, then that location is the obvious choice. It's kind of neat that you can go straight into the park from the restaurant! Also, it seems like it is easier to get reservations at the Animal Kingdom location, so that may work to your advantage as well.
Have you looked into character dining? I think I have narrowed down my son's birthday dinner to
Chef Mickey's. He loves all of the characters and has really gotten into asking for their autograph.
Does your child like dinosaurs? You can check out
T-Rex Cafe, which is owned by the same company as Rainforest Cafe. It's basically the same type of restaurant, with a dinosaur theme. There is a meteor shower every 30 minutes instead of a thunder storm.
All table service restaurant highly suggest advance reservations. Please call (407) WDW-DINE to book your dining reservations when you have narrowed it down.
Have a magical day!
Leslie