You can cast SiteMap node to CmsSiteMapNode and then access the child nodes using ChildNodes property of SiteMapNodeColllection, ParentNode of CmsSiteMapNode and IcmsUrlContext Parent property.
Currently you have the parents. You need to chenck whether threre are any child nodes and bind them. If you get stuck it will be great to share some code with us. This will help us to figure out wthat the exact problem is.
the Telerik team