Thank you for your query,
Your child seems to have Gastroenteritis
mostly viral or a bacterial one.
Maintain her hydration by giving sips of oral rehydration solutions as dehydration
and decreased oral intake may lead bad odor of mouth.
You can ask her to do salt water or betadine mouth gargling.
Give plenty of oral fluids to drink.
And one probiotic may be added like lactobacillus
Consult your Pediatrician
to assess her hydration and for further treatment.
Hope I have answered your query.