defer-concat.cs

var parents = new[]
{
    new
    {
        name = "stu",
        children = new[]
        {
            new {name = "john"},
            new {name = "audrey"},
            new {name = "eustace"}
        }
    },
    new
    {
        name = "bob",
        children = new[]
        {
            new {name = "seymour"},
            new {name = "andre"},
            new {name = "pascal"}
        }
    }
};

parents.ToObservable().Select(parent =>
{
    return Observable.Defer(() =>
    {
        Console.WriteLine("parent {0}", parent.name);
        return Observable.ToObservable(parent.children);
    });
})
.Concat()
.Subscribe(child => Console.WriteLine("child {0}", child.name));