This is hard to tell - it shouldn't in theory.
But as that article pointed out, a small percentage of people will react to prolonged exposure to properly sanitized and maintained water.
So first just try Vitamin C cream (any cheap kind is fine) before/after they swim...it neutralizes chlorine.
That will likely nail it.
If not, you could try further investigation:
Out of your family, 2 children get abdomen rashes after swimming, but not everyone. Right? (Any chance two of them are reacting to sunscreen?)
The rashes are small dots (like pseudomas...otherwise known as hot tub itch, but right away, not 24 hrs later) right? (You should not have pseudomas if you're maintaining TFP levels.)
Do these children have any other allergies or sensitivities?
Is the rash area related to their swim suit coverage area at all?
If so, what are you using in the laundry?
This is a total outlier so don't go dumping your water just yet, but there are a few folks on the planet who react to borax -- you normally find this out when they react to clothing washed with borax. Its rare, and ironic, because is the majority of cases, borax/boric acid actually improves skin condition and is used in things lie bath bombs, skin conditioners, etc., where it acts as a mild fungicide. If you suspect it, experiment by washing a piece of clothing in borax, and then having them wear it when they don't have a rash. If no rash, that rules out borax allergy.)
I should add that I've had eczema since childhood, plus a weird form called dishydrotic eczema. My eczema has been COMPLETELY absent since getting this pool and using TFP parameters, including borates at 50 ppm. So in my case, TFP water has the complete opposite effect than what your kids are experiencing
