Answered Multi-Select Listbox of Child Resources?

What about getTree? https://rtfm.modx.com/revolution/2.x/developing-in-modx/other-development-resources/class-reference/modx/modx.gettree

Two other options to consider:
1. Non-nested though you can use type-ahead + multi-select: https://gist.github.com/silentworks/3608022
2. Neatly nested resources, no type-ahead or multi-select: https://modx.com/extras/package/pickresource2

PickResource is quite useless if you are using Collections (you can't select resources in that Collection, if I am right).

A processor based solution with filtering, maximum limit of resources etc.: https://modx.com/extras/package/superboxselect