Your sermon must explain why the Israelites' request for a king was sinful and also why the Lord granted their request. It must be a good opportunity to show how idolatry is about more than simply worshipping carved images. You must also show how the Lord can use our sin to accomplish good whilst emphasizing that it is not his desire for us to sin but to submit to his commandments.
1 When Samuel became old, he made his sons judges over Israel.