interface ISecuredItem<TChildType> where TChildType : class, ISecuredItem<TChildType>
{
    SecurityDescriptor Descriptor { get; }
    IEnumerable<TChildType> Children { get; }
}

class RegistryKey : ISecuredItem<RegistryKey>
{
    public SecurityDescriptor Descriptor { get; private set; }
    public IEnumerable<RegistryKey> Children { get; }
}

public interface ISecuredItem<TChildType>
    where TChildType : ISecuredItem<TChildType>
{
    SecurityDescriptor Descriptor { get; }
    IEnumerable<TChildType> Children { get; }
}

public interface StubImplemtation : ISecuredItem<StubImplemtation> { }

var securedItemStub = new StubISecuredItem<StubImplemtation>
                          {
                              ChildrenGet = () => new List<StubImplemtation>(),
                              DescriptorGet = () => new SecurityDescriptor()
                          };

var children = securedItemStub.ChildrenGet();
var descriptor = securedItemStub.DescriptorGet();