There could be several causes.
May be the child has gastroenteritis, diet probably is poor or inadequate, bloated abdomen after food, anguish due to …adults home violence etc. other emotional and physical states to take into consideration. Excess of antibiotics.
Think of these remedies
CARICA PAPAYA: The child has aversion to food, vomits and stools with undigested food. Child pale, and weak.
CHELLIDONIUM: Pale, weak with no hunger, Yellow eyes, coated tongue, pale stool, dark vomiting with sensibility in
the right hypocondrio, especially liver sensibility.
FERRUM METALLICUM: Child with no hunger at all, pale, and anemic.
PULSATILLA: Little girls with anorexia, with intolerance to fatty food, never have thirst.
Do not forget the constitutional remedy, but in the mean time a touch of the correct intermediate remedy will help the child to recover, the child reveals the hidden latent trace marks of its miasmatic ancestors and its own personality and until the child gets his constitutional will continue manifesting acute exacerbation of the chronic miasm manifestations that flow out of a particular miasm (genetic inheritance).