What can a child be allergic to? How can I regulate my diet, I don't e…

What can a child be allergic to? How can I regulate my diet, I don't eat sweets, eggs, chicken, citrus fruits either, although after childbirth I ate and nothing happened, but now there is rash on the face and cough in the child, a dermatologist said to send stool for analysis, while waiting I would like to know