What would happen if you hit a child with an adult dosage epi pen ?

If by "hit" you: Mean inject, it is dangerous to give a child any medication that is not prescribed by a physician for a specific medical indication. Adult doses of any medication can be dangerous for children. It is also dangerous to hit the child physically with the pen. Why would you even consider doing any to these? Don't you have anything more constructive to think about?

Side effects: This would depend on the size of the kid and their basic state of health. Epi, or epinephrine is the adrenaline our body makes as a stress response.It speeds up the heart rate, elevates the blood pressure, triggers some changes in metabolism, etc.It doubles the side effect intensity without increasing the beneficial effect, if this were done for an asthma attach or allergic reaction.