How do I erase data on uncharged old iPad and iPhone 6?

There’s no way to erase them while they can’t power on. The iPhone 6 uses a Lightning cable; the iPad may use either Lightning or the older 30-pin connector.

Once charged, you can erase them through Settings or restore them with a Mac or PC. If the charging ports or batteries have failed, they’ll need service long enough to power on before the data can be erased. Don’t sell, donate, or recycle them until then.

Since they were inherited, they may also be protected by the previous owner’s Apple Account. If the owner has passed away, Apple explains the access and Activation Lock process here: Request access to a deceased family member’s Apple Account - Apple Support

iPads use an encrypted file-system that can only be accessed using the correct encryption keys - the only copy of these unique encryption keys being stored within the iPad's security chip (Secure Enclave). Entering the correct device Passcode unlocks the the Secure Enclave and releases the encryption keys to the Operating System.

"Wiping" your iPad instructs the Secure Enclave to destroy the encryption keys - this being known as a crypto-erase. Once erased, all locally stored data is permanently beyond reach and cannot be recovered.

Following this process ensures that your iPad is properly erased prior to disposal:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Unless you can power-in the iPad and enter the correct Passcode, all data remains encrypted and beyond reach. Not even Apple can recover data from the filesystem.

If the iPad can be powered-on and erased, unless the associated Activation Lock was cleared by the prior owner of the device, the device will remain unusable.

Activation Lock

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

If you have the necessary documentation, Apple have recently introduced a new portal though which you may be able to request release of an Activation Lock:

How to remove Activation Lock - Apple Support & start an Activation Lock support request.