Dear A.,
Thrush is when the beneficial bacteria in the system are wiped out by the antibiotics and then a fungus called candida takes over. The only thing I have found that works is Nutritional Brewers Yeast Flakes manufactured by a company called KAL. The containers are cheap approx 13.00 which is a large supply. The other thing is scidophilous which is the liquid that is on top of yogurt. These two things are crucial in correcting the bacterial (good bacteria) in the digestive tract. I do not know how you would get it into the baby unless you took large amounts and then she got it through your milk. The ascidophilous can be given to her right in her mouth by breaking open the capsule. I did both with my youngest who is prone to this stuff. She is now five and still takes it.
Keep in mind that to correct this natural takes about four months to fully break the cycle. I had this as an adult did special diets, the prescriptions that were expensive and nothing worked. Then I did the two things that I list above and it took fours months to fully and completely be gone. Just for safe measure I do take the yeast regularly but I no longer have any of the symtoms, ever! I had them all for years, bladder infections, yeast infections, drove me mad!
I promise you these things work!
Good Luck.
D.