The template you’re using should be fine. If you leave out the &tpl parameter, Ditto will use its own internal default template (getResources will not -- it dumps all the fields without a &tpl parameter).
The publishedon field should hold the last time the document changed from unpublished to published. It should be set automatically for every published document.
I suggested that you unpublish and then publish every document, but Ditto seems to have trouble even if the docs are unpublished.
I’d suggest switching to getResources with the same tag and tpl:
[[!getResources? &parents=`12` &tpl=`tplNews` &orderBy=`publishedon ASC` ]]
or
[[!getResources? &parents=`12` &tpl=`tplNews` &sortBy=`publishedon` &sortDir=`ASC` ]]
Both of these work fine for me using your tplNews chunk. Be careful of the spelling of the snippet, the chunk and the parameters. They’re case-sensitive.