The following Facebook Graph API v2.6 query:
/{{POST_ID}}/?fields=comments.summary(true)&access_token={{ACCESS_TOKEN}}
will get the comments (with theirs IDs) for the specified Post. Then the query
/{{COMMENT_ID}}/?fields=comments.summary(true)&access_token={{ACCESS_TOKEN}}
will get the replies for the specified Comment.
Is there a way to get both the comments and their replies for given Post with one Graph API query?