Based on the error message in the screenshot, the error is returned because the GetRowByKeyValue return null. My hunch is because the data type in the ArrayList and the KeyValues of the parent row does not match. Have you tried using the object of the ChildKey item without casting the item to String?

Or you could use the RestoreExpandedChildRows="true" property in the LayoutSetting so the child row will be expanded and you could retrieve the child row info using the ChildKey variable:

//Acquire the selected child row
WebGrid1.GetTableByName("childTableName").Rows.GetRowByKeyValue(ChildKey)
//Acquire the selected child parent row
WebGrid1.GetTableByName("childTableName").Rows.GetRowByKeyValue(ChildKey).Parent

The workaround does work. We could force the WebTextEditor to HTML view and back to design view to display the toolbar. Here is the snippet to change the WebTextEditor view during WebTab after tab changed client side event:

function WebTab1_OnAfterTabChanged(controlId, activeTab, previousTab)
{
	var WebTab1 = ISGetObject(controlId);
	if (activeTab.Name == "TextEditorInlineContent") {
	    var textEditor = ISGetObject("WebTextEditor1");
	    textEditor.SwitchToHtmlView();
	    textEditor.SwitchToDesignView();
	  
	}
	
	return true;
}

The sample is showing selfreferencing and hierarchical WebGrid. However the self referencing is done on the Root table. If you would like to have the self referencing in the child table, it could be done using the similar technique since our self reference setting is done on the table level.

You could also set the loading mode to Load on Demand whether you are using self referencing in the Parent or Child table. Did you encounter any issue using heirarchical and selfreference setting when setting the selfreference in the child table?