Can you rephrase the question as i dont understand what the issue is.
Youve posted what looks like parameterised .json request payload and you state you want to create 1 application with 2 child records but i dont know what the "application" nor "child records means in this context.
If you remember when describing your issue forum members are not cognizant od your projects functionality or requirements so try and explain what you need from that perspective that will only help.
if this helped answer the post, could you please mark it as 'solved'? Also if you consider whether the title of your post is relevant? Perhaps if the post is solved, it might make sense to update the Subject header field of the post to something more descriptive? This will help people when searching for problems. Ta