I'm not sure I completely understand your question, but at least part of your answer is in the documentation:
https://gpeasy.com/Docs/Navigation_Menus (Josh, can you troubleshoot the image links?)
You can easily add a menu containing just the children or sibling pages by using the "levels". This will include ALL the pages that fit the criteria.
If you want a fixed subset of those pages, you'll have to create another custom menu.
If you want a random subset of those pages, you'll need to write a script. You can write a php script, but it may be easier to just use jQuery to remove some of the links from the full menu.
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