LINQ to SQL using VB, parent child tables, results filtered by value of first sorted child

Greetings Experts,

How can I use Visual Basic and LINQ to query a Parent Table for just those rows which have Child Table rows with a certain value in a Child Table column OR no Child Table rows at all?

In my scenario, I am logging support messages to a SQL table "tblSupport" (they arrive via a web service from a thick client) at my web site.  There are five columns in this Parent table;
ID (BigInt, Primary Key)
ZuluDateTime (DateTime)
AccountNumber (VarChar(MAX))
Type (VarChar(MAX))
Message (VarChar(MAX))

I log my replies (there may be several) to these messages in a second table "tblSupportReply".  This Child table also has five columns;
ID (BigInt, Primary Key)
ZuluDateTime (DateTime)
Status (VarChar(MAX), contains either "open" or "closed")
IdSupport (BigInt, FK to tblSupport.ID)
ReplyMessage (VarChar(MAX))

I have used the O/R Designer and can query the Parent Table for the "new" (no children) messages as follows;
        'LINQ - get records from tblSupport with no replies (parents without children)
        Dim db As New MyDatabaseDataContext()
        Dim qSupportMessagesNew = From m In db.tblSupports _
                               Where Not m.tblSupportReplies.Any()

How can I also return the Parent rows which have their newest Child row (sort by tblSupportReply.ZuluDateTime) with tblSupportReply.Status = "open"?

My goal is to bind the result to a Gridview, which would list all messages which need my attention (either new or open).

Thank you for taking the time to review this question.