water wings

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural A pair of inflatable waterproof bags designed so that one can be attached to each arm, especially of a child learning to swim, to provide buoyancy.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Plural form of water wing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a life preserver consisting of a connected pair of inflatable bags that fit under a person's arms and provide buoyancy; used by children learning to swim