You do not need MDM for supervising your iPads. Supervision can be done either by using Apple DEP within the Apple business manager or by using the Apple Configurator 2 method.
If you have purchased your iPads from Apple or it’s authorised reseller or carrier, you can add it to your DEP and it will be supervised. On the other hand, if you have purchased iPads off the shelf, you can supervise it using a Mac device, Apple configurator 2 ( an Apple app) and a lightning USB cable.
Supervision using DEP is done over the air, without actually having the device with you whereas with the Apple configurator 2 method, you have to physically supervise each device. Supervision is the first step to managing the device and for advanced functions and enhanced control over the device behaviour and functionalities, you need an MDM.