Children's bikes are usually sized by age range, a simple indicator at best. Height and inside leg length are much more reliable guides. As you want the distance from sit bones to foot soles, simply subtract sitting height from standing height (wearing just socks, not shoes). Get your child to stand and then sit (legs straight out in front) with their back to the wall and measure to the floor from a book or rule resting on their head. otherwise, you can get them to stand in socks against the wall and place a small hardback book between their legs and slide it up the wall, stop at the point as if they are sat on a saddle. Hold the book still and mark where its top touches the wall.
The bike's weight is also a big consideration. The lighter it is the easier it is to push, pedal and control.
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