To reproduce: please follow the steps illustrated in the attached gif file. Workaround: add pages as follows: Click the Smart Tag of RadPageView and then click the Add Page link five times. Additional information how to get started with RadPageView is available in the following help article: https://docs.telerik.com/devtools/winforms/pageview/stripview/getting-started
Adding children to StripViewItemLayout is not a suitable approach. You can add pages via radPageView smart tag by clicking "Add page" menu item. Therefore we have decided to hide the children collection from the editor.