Use the model-specific fit range
Check height, inseam, and minimum saddle-height guidance for the exact bicycle. Guardian, for example, offers different 20-inch frame sizes with different fit ranges, showing why wheel diameter alone is incomplete.
Match gearing to ability and terrain
A simple drivetrain can reduce control complexity for a newer rider, while gears can help on hills and longer rides when the child is ready to use them. Compare the actual terrain and maintenance needs.
Check controls and total weight
Confirm that brake levers, grips, cranks, pedals, and shifters are child-appropriate and reachable. Compare complete-bike weight consistently, but do not trade away correct fit, braking, or durability for a number.
Review setup, warranty, and support
Ask what assembly remains, which tools and instructions are included, where service is available, how replacement parts are obtained, and what the documented return and warranty terms cover.