The most reliable way of getting the right kids bike size, is to measure inside leg and arm length as there is a large spread of “normal” leg lengths & heights for each age.

Please take the time to measure the rider’s inside leg. 
It’s easy to measure:

Stand the child in socks against a wall
Place a small book between their legs, as high as is comfortable
Ask the child to move away and measure from the top of the book to the floor.

Once you have measured the inside leg, use this table to find the right kids bike sizes. Note we give the MINIMUM inside leg length for each bike, with the saddle at its lowest setting. At this height, the child will safely and comfortably be able to get their feet on the ground. 

Of course all children have different length arms and torsos too, so to ensure a comfortable riding position, the handlebar height can be tailored, and saddles can be slid backwards and forwards on their rails. (These adjustments are easy to do and just need an allen key or we will be happy to do it as well.).