Have been using my deceased son-in-law's IPad mini for 7 years without need of password. Today I can't get on without activating with id and password which we don't have. How do I activate the IPad so I can use it again?

After seven years, you have presumably triggered an Activation Lock. If the iPad was not properly prepared for reuse by a new User, it is perhaps surprising that in this time you have not previously encounter issues.

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

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, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.

If you have the necessary documentation, Apple have recently introduced a 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.