What was Paul's letter to the Philippians about for kids? A Goggle search...
Paul wrote letters to groups of people about specific things that those particular groups of people needed to hear. Who were the people? What was Paul writing and why?
Where did the people live from a cultural/political/social perspective? If you were a child in Philippi listening to someone read Paul's letter would it mean anything to you?
Find out what you can about Philippi and what was happening at that time in history. What specific events or practices affected Jewish and Gentile believers? Their children? Research! How would the things Paul said affect the grown-ups? How would they affect the children?
Think about someone standing up in church and reading a letter from a teacher who visited once or however many times Paul had come to them previously. Are you meeting in someone's home? Are you hiding? Are families there, not just grown-ups? Think about the situation, the circumstances.
Or without any research, read the letter in the NIV Readers' Version. Imagine yourself a child listening. What do you hear God saying to you? Imagine that as a child you weren't there but your parents came home and you heard them talking. What would they be saying?
Read it to children a little each week in class or a little each day at home. What do they hear? What's relevant to them? Ask them!