Ok, this works
<!-- IF categories.children.length --> <!-- BEGIN children --> <!-- IF @first -->(Sub-forums: <!-- ENDIF @first --> <a href="\{relative_path\}/category/{categories.children.slug}" itemprop="url">{categories.children.name}</a> <!-- IF !@last --> · <!-- ENDIF !@last --> <!-- IF @last -->)<!-- ENDIF @last --> <!-- END children --> <!-- ENDIF categories.children.length -->Just put it somewhere between <!-- BEGIN categories --> and <!--END categories --> in home.tpl
And take both \ from {relative_path}.. it seems that nodebb parses it... so I had to type...