My child has food allergies. What is the best way to explain it to the other kids?

Dangerous. Tell them that he is sensitive to certain foods and he has to avoid them , otherwise it will make him really sick! he has to bring a special injection kit in case this happen to save his life.
They need to know... Food allergies can be fatal. Any siblings, caregivers, teachers, or other responsibile contacts need to know the foods that need to be avoided, how to read labels, how to use an epipen, (epinephrine) etc. Everyone needs to speak up to question the cooks, food servers, etc. About ingredients in dishes. When old enough, your child needs to learn all of this as well.