My son and his family and my husband and I are on a family plan with ATT. Both kids ace hand-me-down ipads. A few months ago we were getting the text messages that we had gone over our data allotment.
Now, the majority of the time the iPads are connected to wifi and our phone usage had never been that high. The culprit was my granddaughter's iPad. She loves to watch Amazon Prime videos. Her iPad would show it was connected to the network but in reality it had dropped the connection, hence using her cellular connection to stream.
We solved our problem by turning off the cellular data while at home. It seems some iPads have an issue with staying connected to some routers. But the wifi symbol would still be displayed.
I know it's bit of a pain and we shouldn't have to do it but I suggest leaving your cell connection off while at home, just to be sure.