Honestly, the baby just kicks back in their stroller & chills. We've taken several babies/toddlers & it is always great. This past year we went in September & we bought this:
http://www.walmart.com/catalog/product.do?product_id=4910778
It was very helpful. This past Sept we went with a 6 month old, a 5 yr old & a 6 yr old. We packed a few diapers, a pack a wipes, an extra outfit, a thin blanket (for nursing & diaper changes), a bottle of antibacterial stuff, ponchos, a baby carrier (Ergo is a good name for a toddler, you can wear it on your back) and the stroller fan & stroller. That was it.
A few days we even stayed ALL day from 7 am to post-fireworks (9 or 10 pm) and had no meltdowns. We just went at our own place, ate at indoor sit down places, and had some sleep in days.
My trip report below shows my packing list & photos of all we did & where we ate, etc. You might find it helpful.
Here's the baby just chillin':
We are in the shade (I'm laying on "luggage") outside Expedition Everest waiting for Daddy & the big girls to get off. THen Daddy laid on the fake luggage while I took the girls back on EE with the baby swap. Everyone was happy. Daddy & I both had a turn laying the shade. The baby was chillin' & people watching. THe girls got to ride EE twice in a row with no wait (we had gotten Fast Passes & then a baby swap.)
Here's our eldest from a WDW trip 7 years ago taking a snooze while we ate lunch.
And here is my middle girl (6 years ago) in a baby carrier obviously not suffering too much. I believe we were waiting to go on Dumbo? Maybe?
Please ask me any specific questions you might have. I disagree that the older a kid is the better Disney is. I LOVED taking my babies. It is always my favorite stage. You really get to kick back & suck in all the WDW atmosphere. You will find your 6 year old wants to race about & is only ride-focused. But because of the baby you will be able to say, "No, let's take it slow," which is the best way.
We also found this past trip that there were times when dh & I split up & one took the baby & one took the big girls so they could race around being ride-focused. It was a lot of fun to race about with the big girls, but I certainly enjoyed sitting in the shade eating my Dole Whip while the baby chilled.