Thank you for your query,
My answer to your query may be not in a appropriate time but I wanted to advise you.
Whenever child has loose motions you can start giving oral rehydration solutions to prevent dehydration
may be addes to add gut protective microflora.
But antibiotic for desntery can be started after consulting your Pediatrician
Inadvrtent use of antibiotic may lead to drug resistance
and may not work the previously prescribed antibiotic in present situations.
Hope i have answered your query.