Approximation is a technique used in therapy approaches which are based on older reflex based conceptions of movement control.
The theory states compression through the joint increases extensor tone or promotes stabilization around a joint.
However I am not aware of any research that demonstrates that this actually happens in practice.
Modern approaches to intervention emphasize motor learning and task based practice. Techniques that emphasize sensory input, such as approximation, to normalise tone are not used.
If you tell us a little more about the movement difficulties the child has - perhaps I can give you some task based suggestions for improving the child's function.