Why my ipad's battery is draining very fast even when it is charging?

What does your battery health percentage read? If it's under 85% or so, Apple recommends replacing it. I'm guessing your seven year old battery is reaching the end of its useful life; what you're seeing is typical of a battery that's ready for replacement.

I've been using this analogy lately to explain how it works. Think of your battery's capacity as a jar you can fill with water, representing a full, 100% charge. Now every time you use the battery, you drop a marble in the jar. Over time, 7 years in this case, the jar gets filled with marbles. Now, you can still fill it with water up to 100%, but it's only holding a fraction of the amount it did when it was new. So of course when you start using the battery, it runs out very quickly.

This guide shows how to replace the battery on your iPad.

iPad 6 Wi-Fi Battery Replacement - iFixit Repair Guide