We did something a lot like this for a site we are working on right now. We had set up a custom index and were using it for a faceted search. We needed to be able to restrict search results to the children of the page with the search on it. This is what we did:

Below is what I added to the config/ExamineIndex.config. What is most important here is adding the parentID to the IndexAttributeFields.

<IndexSet SetName="MyCustomIndexSet" IndexPath="~/App_Data/TEMP/ExamineIndexes/CustomContent/">
  <IndexAttributeFields>
    <add Name="id" />
    <add Name="nodeName" />
    <add Name="updateDate" />
    <add Name="writerName" />
    <add Name="path" />
    <add Name="nodeTypeAlias" />
    <add Name="parentID" />
  </IndexAttributeFields>
  <IndexUserFields>
    <add Name="title" />
    <add Name="subTitle" />
    <add Name="description" />
    <add Name="inactive" />
  </IndexUserFields>
  <IncludeNodeTypes>
    <add Name="SomeDocumentType"/>
  </IncludeNodeTypes>
  <ExcludeNodeTypes>
  </ExcludeNodeTypes>
</IndexSet>

When I wanted to perform my search, I add search criteria that restricts my search results to pages with a parentID that matches the id of the current page we are searching from. It might look something like this.

// Note: I have already set containerId up as the id of the current node (the node doing the searching)
var customSearcher = ExamineManager.Instance.SearchProviderCollection["CustomSearcher"];

var searchCriteria = customSearcher.CreateSearchCriteria();

var query = searchCriteria.Field("inactive", "0");
if (containerId > 0)
{
    query.And().Field("parentID", containerId.ToString());
}

var searchResults = customSearcher.Search(query.Compile());

Hope this helps you get started. Let me know if you have any questions.

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