You might want to try a carrier for your 6 month old, and a double for the older kids. There is a thread here about good baby carriers, from a couple of days ago, ones that don't hurt your back.
The nice thing about a carrier is that you can use it in places strollers can't go (lines, most places at Epcot) and the baby can still sleep and be comfortable. It can also go on some rides, like the ones in fantasyland without distrupting the baby.