This is a charade - where two or more parts are joined together in sequence to form the answer.
Look at 'Papa' - this refers to a single letter from the NATO phonetic alphabet. Then 'child's little' gives you a childish way of saying a small word. Put these parts together in order.
Answer: PICKLE | 'Papa' gives P (from NATO phonetic alphabet), and 'child's little' gives ICKLE (childish version of 'little'). P + ICKLE = PICKLE (difficulty).