What makes my 4 year old break out in itchy rash whenever she eats anything with tomatoes?

Food sensitivity: There are many food items that can trigger a skin rash/hives in kids or adults as a form of food allergy. Some are somewhat mild with a rash persisting for up to a few days while the material passes through the intestines. Some could progress to be more intense with cramping pain or swelling of facial features.I would consider avoiding it for the future or have specific testing done to confirm it.

Workup/Mgt: I agree that a food allergy panel called a CAP test is indicated. If the rash occurred particularly within 4 hours of eating the tomatoes, an allergic reaction is most likely. The itchiness as well indicates an allergic reaction. Avoiding the food, and using an antihistamine by mouth is part of the management. I would also suggest a discussion about pediatric epi pen with your physician.