I'll add an important qualification.

If a small rider is only adopting a very upright riding position then they can cope with the current crop of bikes with big wheels and long cranks. In an upright position they don't have too much weight on their hands/shoulders and their knees aren't lifting into their chest. The problems begin when they try to adopt a more aerodynamic position and increase their performance. Thats where the current off the shelf bikes for small riders fail badly.

Anthony