Charging the battery inevitably creates heat - both by the battery and the electronics that regulate the charging process. Similarly, any sort of processing activity also creates heat - the more intense the activity the harder the CPU/GPU must work and the more heat will be generated.
Your iPad will automatically regulate its operation to keep heating within safe design-limits, ensuring that no damage will occur to either the iPad electronics or its battery. In some circumstances, the iPad may take steps to temporarily reduce system performance or reduce the charging rate to keep temperatures within a safe range.
A healthy iPad will come to no harm whatsoever if it is being used while charging. It naturally follows that a full or near fully charged battery will create less internal heat than when actively charging from a low state of charge - during which the battery will potentially be charging at a higher rate than later in charge-cycle.
The overall battery health or longevity of the iPad battery will not negatively impacted if the iPad is being actively used during charging. Likewise, Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery.
When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.
Nominal 100% charging will be restored when a normal charge/discharge routine is restored.
More information about iPad’s battery management can be found here:
About iPad and iPhone Charge Management Feature – Apple Support