LoadObjectWithChildren() is for animated objects and expects an armature and meshes become child's of the armature.
this issue is somewhat discussed here :
https://forum.thegamecreators.com/thread/227776
if you are trying to use blender as your level editor then you are going to have to write an importer.
AGK does not support any kind of "prefab", you could just add a single bone and parent each object to the bone and load it that way, LoadObjectWithChildren will see the bone (armature) as the parent, I am not sure how/if collision stuff would pan out though as AppGameKit is seeing the entire world as a single object and it may mess with default culling etc etc, worth a try though, maybe