Took my nipper to a reserve game about that age. He lasted 20 minutes.
You've probably got the best policy with an iphone / sweets. Maybe set a few tasks like counting the floodlights / players or guessing what will happen next.
First proper game I took the lad to was promotion game against Coventry which was just before his 5th birthday. He has been disappointed by the lacks of goals and pitch invasion ever game since.