What would stop the AllowedChildNodeTypes getting set on the documents types structure? 

I've tried from a fresh installation and none of them are getting set. Everything else seems to be working ok and I have inherited from DocumentTypeBase at the bottom of the inheritance structure.

I have also used them before so know they have worked in the past.

Here is an example of what i've done and cant see anything wrong myself.

[DocumentType(Name = "Product Ranges", IconUrl = "folder.gif",
    Alias = "ProductRanges",
    AllowedTemplates = new string[] { },    
    AllowedChildNodeTypes = new Type[] { typeof(ProductRange) })]
    public class ProductRanges : ContentMaster

  [DocumentType(Name = "Product Range",
        Alias = "ProductRange",
        AllowedTemplates = new string[] { },
        AllowedChildNodeTypes = new Type[] { })]
    public class ProductRange : ContentMaster