It is possible with some DIY skills to connect (solder) the wires from the relay output of the 6113 across the circuit board of a wireless doorbell push so that you have both smart and dumb doorbells combined.
If you want your parents to be able to permanently view the image from the doorbell (rather than just when there's a call) it's not quite so simple. You'll need any iOS or android tablet with Hik-Connect installed. You'll need to log the Hik-Connect app into the account that the doorbell is registered to. You will also have to add the doorbell as a local device to the Hik-Conect app; the reason being that if you just try to watch the feed from the doorbell in Hik-Connect through the cloud account, it will timeout after 5 minutes requiring you to reconnect (as a local connection is direct to the device over WiFi it can run permanently)
As for the device, it's going to need regularly charging. You would be best getting a tablet that supports mag charging, then getting several stands/docks with chargers and put these in key places around the home. That way, when not in use/in hand, they can drop it into the nearest stand where it's still in view. I'd like to be able to recommend the Amazon Fire devices for this but can't....I've found that the call notifications are hit and miss. I'd use an iPad or at least something that has pure Android on it, as the Fire tablets have their own Fire OS that uses older Android as a base. I also recommend contacting the company
https://www.cogniteq.com/, who will help you with the setup. When I had difficulties with the software, I contacted them and did not regret it.