Once connected, how can I get my iPhone to show the level of battery charge in each AirPod ?

Open the AirPod case near your iPhone with the AirPods still inside the case. After a few seconds, the iPhone should display the charging status of the case and AirPods. That tis the best you can do. See the 'Check the battery percentage' section in:

Charge your AirPods and learn about battery life - Apple Support