Hm...it could be the sesame. Try and see if it's that. Otherwise, hold back on the rye - it is not wheat but a type of grain or cereal (like wheat, oat, millet, barley, quinoa, buckwheat, etc).
It is normal for kids to have reaction to certain things, just like us adults. People are not born with a preset list of allergies, they develop them. I had the unfortunate business of developing a bad case of hives and the itchies a few years back and it was due to eating too much calamari. Six months ago, I had contact dermatitis, an allergic reaction to my wedding ring - it would seem that after a while, the metal layers reacted with certain chemicals like soap, bleach, etc and heat causing a reaction with my skin. If I drink too much alcohol and eat calamari, my palms will start to itch and sometimes I'll develop red patches on my arms as well as hives on my palms.