How to add parent work item id in children task?

You can go to workitem's link tab, then add "set parent" link. From which, there is a dialog pops up and you can search for the parent id there and select it. Once this is set, you can always see the parent tag with the parent workitem id and its summary and you can hove on it to view the parent workitem. If you go to the parent workitem's link tab, you will find the Children tag with the children workitems it has.

 Your requirements are not very clear. You can see the parent in the links section. How to create links, see https://rsjazz.wordpress.com/2012/09/19/the-rtc-workitem-link-api-linking-workitems-to-other-elements/