The waterparks don't allow inflated armbands, or inflatable devices of any kind that are not park-supplied (meaning the river tubes.)
So does Disney let you bring in the USCG flotation devices?
I would always suggest a USCG-approved flotation device, with a collar if necessary for a child who is young enough to not reliably be able to right herself. We use our own because it is cloth covered and a bit more comfortable than foam-surfaced, it has been allowed at the water parks, and it is USCG-approved.
My friends have one of these at their home pool. We used it with DS1 and it was great. I will make sure to look for the USCG-approved.
The currently popular new thing is the Stearns PuddleJumper, but I can't tell you if the WDW waterparks allow them. According to Coleman's website, it is "U.S. Coast Guard Approved Type V with Type III performance." It is meant for children weighing 30-50 lbs; if yours is littler it won't be a good option.