Data usage skyrocking this month

I have 4 plans/devices, 1 for each of my kids. For the past 2 years, they have always been under 1GB (except for a few times when we are traveling). I have their phones locked down, with only approved applications, and mobile data turned off on all the apps. Nothing has changed on their phones. However, this month, 4-days-in, they have already used a full 1GB! Has Xfinity changed what counts as "data" - because the usage and behavior on our end has NOT changed, but we are getting charged for data like crazy. At this rate will have to pay $100+ in data usage this month, when normally it's just a single GB.