How do I add my children with age of 13 and 15 to my Google Home?

Hey Ronald, both those accounts are eligible or should be able to set up the devices using their own email accounts -- no invite needed. They can add them to your master home group, whatever that is, says "Ron's House" and locate them in their rooms in Ron's House and set up voice match and personal results. They will need to install the Home app and set up the devices using the setup flow.

 

I don't believe they need an invite.

Family Link, depending on the settings you've locked down for the kids may prevent that, it also might explain why you're unable to invite kids to your home. I don't have kids so have never had the need or reason to set it up or fiddle with those settings, so, I'd truly be lying if I tried to tell you what settings to change. Take a look here.

 

By the way what smart clock? These don't even sound like Google Home or nest devices? If not, there could be some other issue totally unrelated to Google Assistant; just because a device works with Google Assistant or Google accounts doesn't necessarily mean the issue is Google or Google Assistant related. I have a Bose speaker that works with Google Assistant and that other service provided by the place named for a river. But setup and support are handled by Bose or the Bose app. To my knowledge Google does not make smart clocks.

I use the same smart clock myself and works fine with Google Home.

The Google family settings are as much open as possible as the kids are already 13 and 15.

OK, so I would wipe the thought that the devices belong to your kids and set them up as belonging to you in their room(s). Next, I'd turn off Family Link, then send the kids invites. As house manager you can choose to share only certain items, in this case the two clocks. Their invite once sent and received should provide instructions for what they (you) need to do. Once set up turn Family link back on. 

 

I believe Family link is the issue here and settings you use to monitor your kids may block what you can do or can send to prevent them from circumventing any lockdowns, purchases etc. I mean you don't want them changing passwords for online purchases on your credit card, right?

 

 

Then I would set them up as your devices in their rooms if unable to connect them using their accounts.