Hi @raviranjan,
Can you let know the SPA Project core version you are using.
Also, cross check if Adobe CQ JSON Exporter bundle is active and your SPA project code base is importing the same(As evident from Imported Packages section of your SPA project bundle) and resolved correctly to it.
To be specific, this method from HierarchyNodeExporter - https://helpx.adobe.com/experience-manager/6-5/sites/developing/using/reference-materials/javadoc/co...
is responsible for :children in root page model response of SPA project/page created using SPA root page template.