How to create custom Child object under Custom object in Application composer?

I created a Custom Object under standard object Account in Application Composer.

Now i need to create one more child object under my Custom Object. In Application composer , I don't see any option to create any child object under my Custom object.

currently i have :

Account → Custom Object A

i would like to have :

Account → Custom Object A → Custom Object B

I created a 1:M relationship between Account and Custom Object A , in the Import management section Parent source and Source System reference values are not visible in Custom object A load file. Then i created a Custom Object A as child to Account , now i could see Parent Source and Source System Reference in my Custom Object A load file.