This could be either:

Because the rash appeared after starting azithromycin, treat it as a possible drug allergy until a clinician assesses it. Do not give another dose until you have spoken with the prescribing doctor or an urgent-care clinician today. Photograph the rash and note when azithromycin was started, when the rash began, and whether each lesion stays in one place or comes and goes.

Seek emergency care now if there is any breathing difficulty, wheeze, throat tightness, swelling of the lips/tongue/face, faintness, repeated vomiting, marked drowsiness, or rapidly spreading hives. Also go urgently if the rash is painful, blistering, peeling, purple/non-blanching, involves the mouth/eyes/genitals, or is accompanied by renewed high fever. These can indicate a serious medicine reaction, as described by the NHS guidance on severe skin reactions.

If the child is otherwise well and has only an itchy rash, it still needs same-day medical advice. A clinician can decide whether it is likely viral or medicine-related and whether an age- and weight-appropriate antihistamine is suitable. Do not label the child permanently “allergic to all antibiotics” without assessment, but make sure the possible azithromycin reaction is recorded for now.

The photo shows scattered red, itchy-looking patches without obvious blisters or skin peeling, which can fit hives or a mild drug/viral rash. A photograph cannot safely rule out a medicine reaction.

For itching now: if your child has no breathing trouble, facial/lip/tongue swelling, vomiting, faintness, mouth sores, blisters, or skin pain, you can use cetirizine 5 mg by mouth now. For children aged 6 to 11 years, the usual dose is 5 mg twice daily, spaced 10 to 12 hours apart. Check the bottle strength carefully: if liquid is 5 mg/5 mL, give 5 mL for a 5 mg dose. Do not give it if they have already received cetirizine, levocetirizine, loratadine, chlorphenamine/Piriton, diphenhydramine/Benadryl, or another antihistamine today without advice from a clinician or pharmacist. NHS cetirizine dosing

Also:

Go to emergency care immediately if any swelling of face/lips/tongue, trouble breathing/wheeze, throat tightness, repeated vomiting, unusual sleepiness/fainting, or a rash that becomes painful, blistered, peeling, dark-purple/non-fading, or affects the eyes or mouth. Emergency allergy signs

For temporary cooling and itch relief, use plain calamine lotion or an unfragranced moisturizer/emollient. Apply a thin layer gently 2 to 3 times a day. A cool damp cloth for 10 to 15 minutes can help first.

Avoid:

Calamine can feel soothing, though the oral antihistamine is the main treatment for hives. NHS pediatric urticaria advice

Since this began after azithromycin, still do not give further azithromycin until the child is reviewed today. Seek emergency help immediately for lip/face/tongue swelling, wheeze or breathing difficulty, vomiting, drowsiness/collapse, blisters, peeling skin, or mouth/eye involvement.