DocumentHelper documentation

Hello Kentico support,

I am working on a fairly complex formcontrol. I really need it to be as efficient as possible and that's why I am very eager to user the new DocumentHelper mechanisms.

I am trying to retrieve the direct children of a related-document with a certain (page)type. I know how to do it with two queries (all values within brackets ([ ]) are variables):

MultiDocumentQuery relatedDocuments = DocumentHelper.GetDocuments().InRelationWith([GUID],[RelationShipName]);`

Then for each TreeNode relatedDocument in relatedDocuments:

DocumentQuery relatedChildren = DocumentHelper.GetDocuments ([ChildPageType]).Path (relatedDocument.NodeAliasPath, PathTypeEnum.Children);

The relatedChildren have custom fields / columns and I need them to be accessible (hence the ChildPageType necessity).

I've looked into the documentation found at: [http://devnet.kentico.com/articles/kentico-8-technology-documentquery-api] and the forums: [https://devnet.kentico.com/forums/f58/fp3/t29998/retrieving-data-from-the-database], but I am not able to get the above into one (1) query.

Any help?