You'll probably need to create promotion codes unique to the user on the fly. Example of this as follows using CBSubs SQL Actions.

www.joomlapolis.com/forum/255-developer-members-support/222032-solved-family-subscription#231056

1. How do I make the child subscriptions expire on the same day as the parent subscription.

You don't unless you use CBSubs SQL Actions to force them to expire via database query, but I won't help with this usage as it's invalid and will fire no CBSubs triggers causing none of the integrations to work for whatever you expire via database. Best I can suggest is ensure both parent and child have the same durations so they'll expire roughly at the same time. We've plans to improve this in a future release so parent and child can be grouped together duration wise.

2. Renew and expire when the parent subscription is renewed or expires but the child subscribers cannot renew their own subscriptions

You can only renew 1 plan at a time. We've plans to improve this in a future release so multiple plans can be purchased and renewed at same time. You can't subscribe to or renew a child plan as is if the parent is expired.

I have looked at auto actions and groupjive cb subs integration but I can't seem to find a solution for this scenario.

Best I can suggest is try using the new Internal Users trigger in CB Auto Action then using the Internal Users URL found in CB Auto Action > Config > Internal to execute the actions with the Internal Users trigger. This will cause the action to execute on every user. You'd then need to add a conditional to check if their group owner is still active and if they're not then expire their subscription. To check if their group owner is still active you could probably use CB Query Field to query the database table _cbsubs_subscriptions to see if they expired or not.

In short there is no simple solution. Best case scenario is develop a custom plugin to do exactly what you're wanting.

It also seems like there are no triggers for plan expiration or subscription for cb subs in cb auto actions, am I wrong?

There are, you'd use the trigger onCPayUserStateChange with a conditional of [var2] to check the status. For example [var2] Equal To A means the plan is active and to continue with the action.