Hi Maria,
Sometimes it's hard to find what you're looking for on the Walt Disney World website, but here is a direct link I am giving you to go right to the spot where you can see exactly what you can do with your
little ones at the Disney Theme Parks. You will see that on the right hand side of this page there are many helpful links listed under the Planning Resources such as Height requirements for attractions, the Magical Beginnings Park Maps (places to visit with little ones), travel tips and suggestions, packing list, etc.... I would study all of this information before going so you have a clear path to follow when you arrive as there is so much to learn (it is better to do as much as possible from home so you are more prepared to just soak in the magic when you get there). Trust me, there will be plenty of things that your 7 month old will enjoy that your 6 year old and 3 (almost 4) year old will love too!
As far as your son`s Birthday, you are going to the right place to celebrate his special day. I suggest you pick up a birthday button for him to wear during your whole trip (not just on his actual birthday), so he will have cast members notice his special celebration for all of the days you will be there. As cast members all throughout WDW notice his button, they will chat with him and wish him a Happy Birthday (maybe even a few surprises along the way if he's lucky). If you are staying in a WDW Resort, you can ask for this button when you check-in, otherwise you can go to any Guest Relations office inside the Theme Parks or at Downtown Disney and they can give you one of these Birthday buttons. I hope he has a magical Birthday at WDW and that you may all have a wonderful trip!
Best wishes,
Brigitta