Girls, the child has diaper rash on his cheeks. We're eating everythin…

Girls, the child has diaper rash on his cheeks. We're eating everything now, but it's very difficult to figure out what exactly appears. We don't give sugar, although I suspect that grandma feeds him sweet cookies when I leave him, but she doesn't admit it. And even the cheeks turn red regardless of whether I left with grandma or not. How do you figure out what causes them to turn red?