What Causes Facial Swelling On Forehead And Nose?

My 2 1/2 year old had mild sun exposure yesterday and this morning has light pink skin and slight facial swelling on the forehead and the bridge of the nose. Diphenhydramine helped a little, but I wanted to know what causes this. I'd also like to know if there are any additional treatments I should try and if there is a more effective method than sunscreen to prevent!