Get your children to a doctor, this needs to be addressed by a professional.
Take them to a dermatologist and also to their pediatrician as soon as possible. They have got to get treatment right away for this rash. Switch up their laundry detergent to something mild like Arm & Hammer with no bleach, and use a fabric softener like Snuggles, use hot water to wash clothes, give it a second rinse. Use Dove soap to bathe them and do not use bath bubbles. Use a chemical-free cleaner for the tub, and buy milder toilet paper, like Cottenelle with Aloe. If the boys wade in a local pool or in a pond or lake, or even sit on the ground next to water, they'll have to stop for a while, and buy new swim trunks for when they go back to that sort of thing.
To help their digestion a little better, which constipation is in the middle of all this, they should drink lots of water daily, and eat fiber foods like whole wheat grains and dark green salads, every day. You and your spouse or a good friend need to go together and talk to every adult or older kids who are around them concerning this rash, asking them carefully about how they spend their time with them, to see what your boys' activities are when you're not around, even check out the day care facility if they go to one.