When the weather is hot, a small piece of red like a rash appears on the child's face. Not if it's not hot What happened?

The disease you are talking about should be miliaria, also known as "hot miliaria" and "red miliaria". It is caused by excessive sweating and poor evaporation of sweat in a hot and stuffy environment, which leads to the blockage, rupture and infiltration of sweat into surrounding tissues. The main manifestations are small papules and blisters, which are more common in summer, children with poor perspiration regulation and long-term bedridden patients. The treatment of this situation, if necessary, can be given topical talcum powder and calamine lotion.