Key cautions & base contact

The IM and IV/IO epinephrine solutions have different concentrations and maximum doses.
Document prearrival epinephrine and diphenhydramine to avoid treating those doses as if they never occurred.

Classify the reaction

Stable rash/urticaria alone has a different branch from angioedema or systemic anaphylaxis; SpO₂ below 95% moves the patient to the anaphylaxis branch.
Anaphylaxis findings include hypotension, wheeze, hypoxia, stridor, intraoral swelling, GI symptoms, or impending obstruction.
For isolated facial/cervical angioedema, the source lists one 0.01 mg/kg IM epinephrine dose (maximum 0.5 mg), held if an auto-injector was already given, plus oxygen and diphenhydramine as specified.

Give epinephrine 0.01 mg/kg IM of 1 mg/mL into the lateral thigh; maximum 0.5 mg. Count prior auto-injector treatment as the first dose.
Provide oxygen as tolerated. For hypotension, give normal saline 20 mL/kg IV/IO, maximum 250 mL per bolus, and contact base; may repeat twice for 3 total boluses.
For wheeze or room-air hypoxia below 95%, give continuous albuterol 5 mg/6 mL as tolerated.

With continued hypotension, respiratory distress, or impending obstruction, a second dose may be 0.01 mg/kg IM of 1 mg/mL, maximum 0.5 mg; OR 0.01 mg/kg IV/IO of 0.1 mg/mL, maximum 0.3 mg.
After initial epinephrine, give diphenhydramine 1 mg/kg IM/IV/IO once, maximum 50 mg, unless already taken before EMS arrival.

Contact base (pediatric-capable preferred) and provide ALS escort to the designated pediatric-capable or other ERC.
Reassess airway, respiratory effort, perfusion, and response repeatedly. Even a resolving reaction after self-administered epinephrine needs ALS evaluation and transport as described by the source.