Try this code. Put it into the ThisDocument Module to have it run automatically when you exit any content control.
Code:
Private Sub Document_ContentControlOnExit(ByVal myCC As ContentControl, Cancel As Boolean)
Dim aCC As ContentControl
If myCC.Title = "Checkbox" Then
For Each aCC In ActiveDocument.SelectContentControlsByTitle("ContentControl")
aCC.Range.Font.Hidden = Not myCC.Checked
aCC.Range.Paragraphs.Last.Range.Font.Hidden = Not myCC.Checked
Next aCC
End If
End Sub