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It should be in context of the content. Most of these child posts are just variations of the elements present and the situation (which is almost always sexual).
Considering the experience obtained from game CG's, I'll try to express it in a table according to content.

Content/Variations allowed
Content | 1 / 2 / 3+

Different
background/ | Y / Y / Y
characters present

Different | Y / Y / X
scenery

Clothes | Y / Y / X

Underwear | Y / X / X

Situation | Y / X / X
(sexual)

Sexual | X / X / X
elements(organs, liquids)

Censorship | Y / X / X
present

I hope I cover it all. Variatons that fall under X should be considered Useless variation.
I'd like to add a question: A set of images should be parented to only one post, by not allowing children of children? As, I've seen, the large quantity of variations can comprehend situations in which users do this.