Start with the school office, not the classroom teacher. Ask for the written allergy or medical policy before you enroll. Find out whether the canteen is run in house or outsourced to a caterer; one kitchen is far easier to brief than ten food stalls. Many international schools in Bali keep a nurse on site and log allergies in a system teachers and kitchen staff can check. Put the allergy in writing in both English and Indonesian, and ask that it travel with your child to excursions and class parties, not just sit in a drawer.
Restaurants are a different challenge, because Indonesian cooking leans on shrimp paste and soy, often in dishes that don't look like they contain either. A laminated card in Indonesian naming the allergy and the exact ingredients to avoid works better than explaining it in the moment, especially with staff who are busy or new. Smaller warungs cook most dishes to order, so a clear request to leave out a sauce or swap an oil is usually possible, but shared fryers and woks mean cross-contact is a real risk even when the dish itself sounds safe. At family-friendly food courts and markets in Bali, where several stalls share one seating area, it helps to pick one stall you trust and stick with it rather than mixing dishes from different kitchens.
Pack whatever your child's doctor has prescribed, whether that is antihistamines or an epinephrine auto-injector, and make sure at least one adult at school knows how and when to use it. Teach older children to ask about ingredients themselves and to say no to food offered by friends, since birthday treats and shared snacks are a common source of accidental exposure. Parents already face a similar judgment call with a child's fever in Bali needing a doctor, and a food reaction deserves the same quick decision: swelling of the face or throat, or sudden trouble breathing, means heading straight to a clinic rather than waiting to see if it passes. Before you travel, ask your child's regular doctor for a written allergy action plan, and find a clinic or pediatrician in Bali you can call the moment something feels wrong.
This is general information, not medical advice. For anything specific to your child, please consult a doctor or a travel-health clinic.
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