Mine have strollers for sure up to age 5, then on a case by case basis after that...LOL
Our dd just turned 5 on our trip this past October. She didnt ride in it all day by any means, but it is nice to have available for her to jump in and out of is she wanted...or if we needed to get across the park in a hurry for a FP return, etc.
It also comes in handy at the end of the day when her little legs are tired.
WDW is a TON of walking...and none of us are used to those kind of miles on our legs

We took a lightweight Mac Volo for our middle DD when she was 6, and will probably do the same for our youngest dd on our next trip. She will be 6 as well. She is and has always been a tiny peanut so she fits in the Mac easily.
I would say that since you have a smaller child, go ahead and take a double or two singles and play it by ear for the older child.
Just re-read your post and noticed you have a BJCM double. Awesome stroller. We were still using our single BJCM for youngest dd on this past trip. Will probably move to the Mac for our next trip...but the BJCM is great and there are tons of them in WDW. Just be sure to mark yours as there will be so many just like yours.
I always have DH make signs at work and laminate them, then punch holes in it and attatch it to the stroller handle with zip ties. This ensures your stroller will not get taken by mistake, and detours anyone wanting to take your stroller as you would have to have scissors to get the zip ties off.