Patrick,
Independently from KML, my “Cesium” plugin for QGIS now publishes data using geojson format.
The question is: are the vector layers in Cesium only flats ? That is: if I switch ON the DTM, my geojson geometry disappears, because it sink under the terrain 
There is a settings to have the geometries moved over the Cesium terrain surface ( as it happens automatically in Google Earth ) ?
Thank you
Roberto