Could be lots of things. Look at your devies. They have tools on there that will tell you how much data each app uses. That will give you the answer. I'm assuming you have iphones
View how much data you're using
To see how much cellular data that you've used, go to Settings > Cellular or Settings > Mobile Data. If you're using an iPad, you might see Settings > Cellular Data instead.
Scroll down to find which apps are using cellular data. If you don't want an app to use cellular data, you can turn it off for that app. When cellular data is off, apps will only use Wi-Fi for data.
To see the cellular data usage for individual System Services, go to Settings > Cellular or Settings > Mobile Data. Then scroll to the bottom of the screen and tap System Services. Cellular data can't be turned on or off for individual System Services.
You can view the data usage statistics for an app from a current period, or view app data statistics for apps that use data when you were roaming. To reset these statistics, go to Settings > Cellular or Settings > Mobile Data, and tap Reset Statistics.