Stop iPad from automatically using iPhone data?

The feature you are describing is called "Instant Hotspot," though you will not see that phrase in the UI of any of your devices. Instant Hotspot allows Macs and iOS devices to use the "Personal Hotspot" feature of one an iOS device to provide internet connectivity to a Mac or another iOS device, without the need to switch on the Personal Hotspot feature on the host iOS device.

In order to use Instant Hotspot, each device must be signed into the same iCloud account. That's it. There is no switch to enable or disable. Just sign into the same account.

It seems impractical to ask your users to use separate iCloud accounts for their phones and iPads... so your best solution may be to call up AT&T and ask that the Personal Hotspot feature be disabled on the phones... thus ensuring that the iPads will stick to their native data connections.

I have the same problem and have been talking to Apple. It is caused by the iPhone and iPad having the same username and password to iCloud. The only way I got around this is by turning off Bluetooth on the iPad, which stopped them creating the hotspot but still allowed my car kit to work on the iPhone.