How to install my developed app into child account in Amazon Fire HD 8 Tablet?

I have developed an Android app in Android Studio (4.1.2) specifically for my two nieces. My brother in law has his account (admin account) and the account of the two girls (managed through parent control) in an **Amazon Fire HD 8 Tablet**. I have managed to enable the development options in the account of my brother in law, I have successfully installed the app in the account of my brother in law but now I am stucked at the point on how to transfer the app to the accounts of both of my nieces. I am aware of this [question](https://android.stackexchange.com/questions/103774/how-can-i-transfer-apps-from-one-profile-to-another-on-my-tablet) , but in this particular case I am not able to see my developed app in the apps than could be transfered to the chosen child's account. **How can I transfer my developed app to the children accounts in the Fire HD 8 Tablet? Is this even possible or does Amazon only allow for certain apps?** Any help is highly appreciated and probably valued with a lot a fun time by the girls :D