Children’s bikes are often sold by age range, yet children of the same age can be very different sizes. Height and inside-leg length are better starting points. Wheel diameter affects both saddle height and reach to the handlebars, so the size on a label does not guarantee a comfortable ride.
In the shop, have the child stand over the frame: there should be clearance above the top tube. Then let them sit on the bike while an adult holds it steady. If the handlebar makes them stretch too far, or the brake levers are difficult to reach and squeeze, that model is a poor fit even if the wheel size looks right.
Bike weight and crank length matter too. A heavy bike is harder for a child to handle, while overly long cranks make pedalling awkward. A short fitting session offers the clearest answer: the child should be able to start, brake and stay in control comfortably.