Stinging insects, Aeroallergens (uncommon), Foods and additives like Milk, egg, peanuts, nuts, soy,
and wheat are the most common agents to cause such dermatitis
and allergies in children.
This triggering factor should be identified and should be avoided.
Treatment includes anti histaminic and glucocorticoids against prescription.
I suggest you to consult pediatrician for further investigations. Take care and regards.