My sister's girls are all allergic to nickel. And they found out early on, when they tried to wear necklaces or pierced ears. They long ago learned what to avoid, and they barely even thought about it anymore.
However, her son kept having an itchy rash on his stomach, near his waist, and it was making him miserable. My sister changed laundry soap. She changed his bath soap. She changed his sheets. She tried a dozen things, but she couldn't figure out what was going on until she realized that it didn't happen when he wore his shirt tucked in. He is allergic too, and was reacting to the metal in the backside of the snaps at the waistband on his jeans or shorts.
They are trying different things to avoid this, such as painting clear nail polish over the metal. But you are right in telling people to suspect metal allergies early on. It seems more ubiquitous than ever.