Are your children's bicycles safe for sale and use? Children's bicycles, with a rear wheel propulsion mechanism and seat height of 435- 635 mm, are designed for kids aged 4 - 8 years. There are various bike designs and wheel diameters. Bikes may or may not come with training wheels. Find out how safe your bike is with the STC Children's Bicycle Testing service, accredited by CNAS & CPSC.