I drew these sketches of my son while he was playing. He was moving all the time and not standing still to pose for me at all. When doing sketches such as these the main goal is to capture the essence of a pose in just a few lines. No rendering. It is a very good exercise for the eye.
As an illustrator it is tempting to want to create beatifully rendered pictures. However it is important to explore freely when sketching, and allowing yourself to make mistakes. Not every sketch will be a masterpiece and that is a good thing. Analysing why a sketch doesn't work and trying again and again will eventually benefit your skills.