Adrenaline is the most important drug for the treatment of an anaphylatic reaction¹
EpiPen is the only Auto-Injector specifically designed to deliver intra-muscular adrenaline in high stress situations, through clothing if necessary

EpiPen is for all patients over 30Kg and delivers 0.3mg of adrenaline
EpiPen Jr. is for patients 15-30Kg and delivers 0.15mg of adrenaline 

Reference

¹ Resuscitation Council (UK)
Emergency treatment of anaphylatic reaction
Guidelines for healthcare providers
Working Group of the Resuscitation Council (UK) January 08