EditableItems["EditableRegionId"]

I use the following macro to get the value

{% CurrentDocument.DocumentContent["editabletext"] |(identity)GlobalAdministrator%}

Its returns the value in page which was enter but not return the value in Child page which need to be inherit from the parent content. Is there any other way to read the content on child page also.

Why not create a doc type specifically for that? Its a much more robust solution than using a document's content. This way you can perform validation and such on it if needed and get the data much easier.

Because our site is almost ready with 2K pages. So changing the doctype is very expensive task now. Is there any way to add a field to existing doc-type with inheriting feature. I know we can add custom fields to doctype. But is there any way to make it inherit for the child document also.

Yes you can add a new field to an existing doc type and have it inherit. You'd have to add the webpart to display it. Then add a page placeholder and create a template with the new content on it that those sub pages would use.

I very rarely use Editable Text webpart because there is really no ability to make your site robust. It doesn't allow you to manage the content very well, its static to that particular page and that's really it.

You are right, I going to get rid of Editable Text Web-part for this.

can you please help me how to configure the Field in doc-type as inherit from parent document.

I am clear about the place holders in this scenario. I have an web-part in page template, Only this is I have to set the Where condition as this place holder value then only the page and its child display the content relative to that field value.