How do I reduce iPhone data usage on my cell phone plan
To conserve data, you can turn off background app refresh on some of your apps (found under settings) or you can go onto everyone's phones and do the same. The feature was designed to update your Facebook, Twitter and other feeds while your not in the app.
See what apps are using the most data (found under Settings > Cellular, then scroll down to Cellular Data Usage) you will be able to see a breakdown of what uses the most data, so you can prevent this problem from occurring again.  
Make sure iCloud Photo Library is off or has the function that only uploads photos on Wi-Fi. 
The App Store has recently been given privileges to update apps automatically, and iTunes can download new content automatically. Turn this off for cell data via Settings > iTunes &amp; App Store > Use Cellular Data.