Hi and welcome,
Your child is suffering from streptococcal pharyngitis
. it is a bacterial infection leading to inflammation of mucous membrane
of the throat and the tonsillar crpts.
It often shows symptoms of cough
, nasal drip , pain and swelling in the throat , difficulty in swallowing and production of phelgm. brownish dark green colored mucus you have mentioned is nothing but phelgm .
Since your daughter is too young i suggest you to consult a pediatrician
and get the clinical evaluation and diagnosis done and then go for treatment.
Till then you can give her soft food , avoid sour and spicy food, avoid citrus juices and cold water , give her distilled or boiled drinking water instead.
Mouth gargling with lukewarm saline water gives soothing effect to the throat.