What's the rule here? Should I group the images under a parent/child relationship?
Posted under General
Parent/child relationships are for images that are themselves related, not for images with the same or similar sources. An artist may post a compilation of their work (over the past month, or their most popular pictures, for example), which may not be related images at all, and they usually wouldn't go in a parent/child group. On the other hand, an artist may make many posts, sometimes across twitter and pixiv, of images that are related and should be grouped together. In that instance parent/child groups are usually used for a small number of very similar images (especially when you've got multiple resolutions of the same image, or slight variants), and pools are used for larger groups and comics where the order of the images matters (people also use parent/child groups for short comics, but if you upload a page in the wrong order you can't re-arrange them).
It's pretty common for multiple images in the same twitter or pixiv posts to be related enough that it makes sense for them to be grouped as parent/child or in a pool, but it's all based on the images themselves.