What can i use on a 10 month old baby with thrush on the mouth?

Depends: Your post does not give your location. In the USA it is common for a doc to just call in a prescription oral solution called nystatin. This is painted on the involved sites with an ear bud or just placed in the mouth. There is an OTC dye called gentian violet that works but it is very messy & permanently stains anything it touches.