Hi there..
thrush is a yeast infection.  Is mom breast feeding?  Doctors will usually prescribe nystatin or diflucan (anti-fungals) for both mom and baby, they will kill off the yeast.  However, yeast infections are caused by an overgrowth of yeast usually due to an imbalance of the natural "good" bacteria in the body.  If natural bacteria (probiotics) are low (often caused by taking antibiotics or steroids) then yeast will grow.  
The best thing to do is go to a health food store or walgreens/cvs and pick up some probiotics.  Get them in capsules so you can open them up and spread them in the babies mouth before eating.  If mom is breastfeeding, she should be taking the probiotics too.  This will restore balance to the bodies natural flora and the yeast will be kept in control by the re-colonization of the good bacteria the body is missing.  
Probiotics are completely safe and natural - as a matter of a fact, yogurt is recommended as a yeast treatment because of the large quantity of probiotics in them.  
Hope this helps, feel free to ask more questions or google probiotics for yeast.  
Lisa