According to this article, children who are overweight or obese should be injected in the lower thigh or calf in order to ensure intramuscular administration of epinephrine.
http://www.familypracticenews.com/news/child-adolescent-medicine/single-article/for-overweight-children-inject-epinephrine-in-lower-thigh/42febcdd9aa45db553217e756756fcee.html