Welcome to BYC. I would test your 6-year old based on the symptoms of the two you lost, before you bring the younger ones in. I like this lab https://www.vetdna.com/about-us, because they do not have reporting requirements. Results go only to you. There is a good symptom checker here...
I agree with @Debbie292d. Also, the root cause could be what @Eggcessive says. It is highly unlikely her issue is related to food. I think her skin looks fungal. I think it is gleet. Do you have monistat? It would not hurt to try it.