Given you familial relationship, you might have adequate information to recover your Father’s account and the devices. If not, Kurt has already provided Apple’s procedure for requesting access to deceased customers accounts.
All of the following information is openly available within Apple’s resources and this Community...
Normally, if the AppleID and associated password were forgotten, they would be recovered here:
https://iforgot.apple.com
To recover AppleID credentials, the owner would will need access to either the primary or “recovery” email addresses that were configured when setting-up the AppleID account - or one of the “trusted” phone numbers associated with the account.
Following recovery of AppleID credentials, the account could be managed and/or the primary email address changed here:
https://appleid.apple.com
If iPad passcode is unknown, the following process, requiring the AppleID credentials to complete, would be followed:
https://support.apple.com/en-gb/HT204306
This article describes the Activation Lock and processes to recover access:
https://support.apple.com/en-us/HT201365