These rashes sound like eczema of some sort.
PoisonAmmonia poisoning
Campylobacter enteritis
Contact dermatitis
E. coli enteritis
Food poisoning
Lead poisoning
Meningococcemia
Methylmercury poisoning
Poison control center - emergency number
Poison ivy - oak - sumac
Poison ivy - oak - sumac rash ivy doesn't fit the location (it would be elsewhere too), but I can't think of anything else that would have contacted your kids in just that location. If you can think of something, the obviously you need to avoid it. (Remember--for something to cause an allergy, it has to touch just the affected area. You don;t get a localized rash from a soap or detergent used all over.)
Treatment for oozy rashes usually involves cool compresses and topical cortisone creams your pediatrician can give you. If the condition persists more than 2-3 days,spreads, or recurs in your other son, you ought to see a dermatologist. If, on the other hand, it just goes away, then I'd just forget about it. Strange things like this happen all the time, and don't need attention unless they persist, spread, or recur.
Best.
Dr. Rockoff