We haven't done SWS yet - going in June 09, but did do the Peru trip on which the youngest child was 6. So here's my 2cents fwiw:
It really depends on your kid. There is a lot of walking on these trips, and you are on the go all day for days in a row. If your youngest is used to keeping up with her older sibs, doesn't need a rest time midday, and is comfortable eating when and what the older folks are eating, then she will probably do fine.
The 6yo on our trip was a trouper, and had already traveled all over the world. She never whined or complained, and kept up with the group very well. One thing that helped, though, was that she had 3 teenage siblings. By midday, one of them was pretty much constantly carrying her. They also worked very hard at keeping her distracted and entertained during any times of waiting, on the bus, etc. I think if the parents had had to do all the work themselves that the teenagers shouldered, it would not have been as relaxing a trip for them. I also think if the teenagers had not done all that work, some of the rest of us might have ended up frustrated with such young child on the trip.
Some of the age rec's are due to the activites that have age and/or weight requirements. For example, the rafting. If your 5yo is tiny, you may want to check with Disney about the restrictions on this trip.
One other thought: the next youngest on our trip was our dd, 10 at the time. So the 6yo did not have an opportunity to make any friends, although everyone was nice to her. It may be worth asking the age range of the kids booked on the departure you are considering, to see if there would be anyone in your dd's range.
I hope this helps without sounding too negative.