Just wanted to chime in about the 6 year old....
We did Disney when my youngest was 9 months old and my oldest was 6.5.
The parks are EXHAUSTING!!! Your 6 yo will likely need to ride, especially as it gets later in the day. We had our P3 for the baby and our Volo for the 6 yo. My DH was very hesitant to bring the Volo, but he was glad I insisted. DH would have had to carry him on his shoulders for hours and that would not have been fun!
Just something to think about...