Hot/humid weather can cause prickly heat in children. Most commonly seen around the face, neck arms, back etc. Overdressing and tight-fitting clothes can also cause them too.
If your child's rash looks like tiny red bumps (with or without small clear fluid), it is most probably prickly heat. Provide cool/sponge bath, use loose/light clothing and apply prickly heat powder (avoid the hot/minty ones).
If it does not fit the prickly heat rash description kindly take your child to see a doctor for proper evaluation and treatment