I wish WDW would be more consistent with their rules - and I wish they wouldn't let kids with those stupid inflatable arm swimmies anywhere near a pool. I've witnessed two incidents at WDW where lifegurads had to jump in to save kids with swimmies that fell off. The parents were nowhere to be seen, either.
A life vest might make it a bit safer for your kid, if it is done up properly and has been used less than the recommended time in water. Older life vests can become damaged by sunlight, become waterlogged by being submerged too long, and can become very dangerous for a child to wear. The life vests you borrow at WDW are lying in the sun all day, and have been used for how long in the pool?