@KRISHKY You can directly add the Page/Pagelist in the service rest rule in the response tab and select the Mapfrom as JSON. The page or pagelist needs to be present in the primary page of the service activity or you can refer the syntax as Page.Property name in the map from key of service rest rule.
Response will be automatically sent as JSON to calling system.
@KRISHKY - You can have a pagelist structure under parent which can be temporary i.e. existing only in the scope of the service activity containing details of the child items. If you map the Parent's page in the response then child details will be sent in the json since the pagelist structure is inside it.