training wheels

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

• noun plural A pair of small wheels attached to the rear axle of a bicycle so that beginning riders can ride without falling over.

from the GNU version of the Collaborative International Dictionary of English.

• a pair of small wheels attached to the rear of a bicycle to keep the bicycle upright; -- used to assist those, especially small children, learning how to ride a bicycle.

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

• noun A pair of small wheels attached on either side of the rear wheel of a child's bicycle so that the bicycle is easier for the child to learn to ride.
• noun figuratively Anything designed to make something easier for a novice.