public void addLinkedChild(ModelKey key) {
        if (assetLoaderKeys.contains(key)) {
            return;
        }
        assetLoaderKeys.add(key);
    }
Add a "linked" child. These are loaded from the assetManager when the
AssetLinkNode is loaded from a binary file.

@param key the ModelKey to add
jme3-core/src/main/java/com/jme3/scene/AssetLinkNode.java