Girls, hello everyone! The child has been turning red in the bottom (m…

Girls, hello everyone! The child has been turning red in the bottom (mucous membrane) since 2 months old. Now he is 1 year and 3 months old, it still hasn't gone away. We have ruled out allergies to powders, diapers. We have seen many pediatricians, dermatologists, but all without results. The redness gets slightly better when I apply tridox. The dermatologist recommended excluding sweets, red meat and dairy, it didn't help. What could this be and what doctor should I go to? Girls from Moscow, please recommend good pediatricians and dermatologists - allergists ??????