If your 9-month-old baby has diarrhea, it is important to focus on preventing dehydration. You can give them a rehydration solution like Pedialyte and consider using probiotics for infants. To help with irritation, you can apply a cream containing zinc oxide. It's essential to monitor your baby's symptoms and consult a doctor if necessary, especially if they have specific concerning signs like blood in stools or persistent fever. Always follow the advice of healthcare professionals to ensure the well-being of your child.