Possibly because of a general increase in fatty covering of western residents since the middle of the 19th century it has been appreciated that some injections intended to be intramuscular are actually subcutaneous. An increase in needle length, but not width, can avoid this, and in children reduce the incidence of an exaggerated local reaction.
Intramuscular injections must be given into an adequate piece of muscle, away from vulnerable structures of which the most widely discussed is the sciatic nerve. The best site in children is the lateral aspect of the thigh. In adults the deltoid muscle is convenient, and for large volumes or variety the gluteal muscles can be reached with a needle longer than is generally expected.
In order to avoid hitting bone in children bunching up the muscle rather than stretching out the skin has been popularised.