In order for probiotics to be effective, you should give them about 3 hours after or before each dose of antibiotics. Once the antibiotic treatment is over, I'd offer probiotics once or twice a day for about two weeks. It's best to use multi-billion-count probiotics.
Also, you can give your chin some alfalfa hay, dry dandelion leaves, and a small amount of oats because they contain naturally-occurring prebiotics. Prebiotics are used by probiotics (for example, preferred food of bifidobacteria are oligosaccharides); they keep probiotics active.