Hi,
Welcome to MODx. I’m not absolutely sure I follow you, but I’ll describe one scenario. Let’s say your Ditto call is in a document, and that its children documents are the ones you’d like to display.
Your Ditto call could begin like this, and could be contained in a template or chunk. It makes "itself" become the root:
[!Ditto? &parents=`[*id*]` ....
Another way is by making a TV (template variable) and assigning it to the template you’ll be using. Then use the TV in your Ditto call, and then each document will allow you to pick the root by entering a number in the TV (once you’ve created a document so you know what the number is.)
[!Ditto? &parents=`[*MyBrandNewTV*]` ....
I hope that made sense and is helpful.
Cheers, Dave