Your ChildTest1 scene is stored in another .tscn file (ChildTest1.tscn) than RootTest.mod.tscn. This means that RootTest will try to load res://CUSTOM_DATA/modsdev/ChildTest1.tscn (which work in Godot because the editor consider that every files of the project are inside res://) but Hotscreen can't find it with this name.
Solution 1 : if you can, all your mod should be 1 single scene. In your example, you can probably delete ChildTest1, go to the ChildTest1 scene, copy all the scene tree, and paste it under VBoxContainer in RootTest scene.
Solution 2 : you can load your other .tscn files in your script (that what my mods example add Collection and add Filter do) Like this :
var scene_for_mod = load("./CUSTOM_DATA/modsdev/ChildTest1.tscn") # there is no res:// so Hotscreen knows its a file path
$VBoxContainer.add_child(scene_for_mod.instantiate())