For a 12-inch bike, which typically targets children aged 2 to 4, the frame material is critical; Hi-Ten steel offers durability and cost-effectiveness, while aluminum alloy provides a lightweight alternative for easier handling. Ensure the braking system includes a rear coaster brake (pedal brake) for intuitive stopping and a front caliper brake to help children learn hand-brake coordination. The tires should be pneumatic (air-filled) for better shock absorption on uneven surfaces, and the training wheels must have reinforced steel brackets to prevent bending during initial learning phases.