I got what looked like poison ivy from a new soap I tried earlier this summer. I used benadryl for mine, but not sure if you can use that on top of Allegra. Even after I stopped the soap, it took about another week for the rash to go away. The clay might help topically or ingested (I don't know what the dosage is for someone that young though). But when my DS has eaten something he's reacted to, I've given him clay (for him it's 1 Tbs. in 6 oz. of water, but he's 10yo), and it's shortened his reaction time. Have you tried olive oil based soaps? I'm thinking of going back to that (and using Dove sensitive skin in the meantime, which so far I haven't reacted to).
Kathy, mother of 3, wife of 1. My new recipe blog: www.kathysrecipebox.wordpress.com (no longer searchable by allergen, but at least it doesn't have a virus!)