Unfortunately, you cannot share in-app subscriptions even through Family Linked Apps. This is not something we can control as developers. You can read more about this here: https://support.google.com/families/answer/6294544
However, you can share your subscription across multiple devices using the “Sign-in” feature.
To share your subscription on another device, please follow the steps -
Open Kiddopia on the device that you purchased it.
Tap on the Settings icon.
Here tap on Sign In and follow the instructions.
Once you have signed in, please follow these steps on all devices on which you wish to share the app using the same account to sign in that you used above.