Absolutely not harmful. You can safely use the iPad while it's on the charger.
Is it OK to play game while charging?
3 Answers. Playing while charging does not damage the battery or phone directly. The only problem is that: battery gets warmer when is charging, and phone also gets hot when playing.
Is it OK to leave iPad plugged in while using it?
It is completely safe to leave your iPad connected to its Power Adapter for extended periods. Whilst connected to the Power Adapter, the battery will charge - and the iPad will draw power combined from both battery and Power Adapter.