The child post are as follows according to my understanding:

Take this post for example:post #937064

When there are only two pictures:
The uncensored or high-definition original image of Fanbox can be used as a parent post:post #937064
The censored or low-quality preview image of pixiv can be used as a child post:post #935787
parent post---->child post1
post #937064->post #935787

When there are multiple pictures corresponding to the parent post and the child post:
post #937064 as a parent post,post #937065post #937066 these as child post.
post #935787 as a child and as a parent,post #935788post #935789 these as child post.

parent post---->child post1---->child post2---->child post3
post #937064->post #935787->post #937065->post #937066

parent post---->child post1---->child post2---->child post3
post #935787->post #935788->post #935789->post #935790

The uncensored or censored who are the child posts,I personally think that the uncensored can be used as the child post.