Hi there, welcome to Healing well. I hope we can provide you with the support you need.
Well, your other two children aren't experiancing any symptoms it's probably not urgent to get them tested. Constipation isn't a CF symptom, unless you have too many creon, which your son isn't on. But loose, greasy, bright yellow or orange stools are usually the case.
I hope everything goes well, and it turns out your daughter is just a carrier of CF.
All the best