Is there a linq-y way to union collection properties of a collection of objects?

Sorry, that title just hurts. I'm wondering if there is a Linq to collections extension method that collapses the following code segment into a single line:

public IEnumerable<Child> GetAllChildren(IEnumerable<Parent> parents){

  var result = new List<Child>();

  foreach(Parent parent in parents)
    foreach(Child child in parent.Children)
      result.Add(child);

  return result;
}

If you can collapse that into a single statement, try it on insane difficulty:

public IEnumerable<Child> GetAllChildren(IEnumerable<Grandparent> nanas){

  var result = new List<Child>();

  foreach(Grandparent papa in nanas)
    foreach(Parent parent in papa.Children)
      foreach(Child child in parent.Children)
        result.Add(child);

  return result;
}

From stackoverflow Will

• This will work:

  public IEnumerable<Child> GetAllChildren(IEnumerable<Parent> parents)
  {
      return from parent in parents
             from child in parent.Children
             select child;
  }
  

  and then this:

  public IEnumerable<Child> GetAllChildren(IEnumerable<Grandparent> nanas)
  {
      return from papa in nanas
             from parent in papa.Children
             from child in parent.Children
             select child;
  }
  

  Note, in this example I'm not actually returning a list, I'm returning an IEnumerable data source that until you start to foreach over it, or similar, won't actually do any processing.

  If you need to return a list, modify each return statement as follows:

      return (from .....
              ...
              select child).ToList();
  

  Will : The return value has to be an IEnumerable, and the easiest way (at least as far as I know) to do that is return a collection that implements IEnumerable. Thanks for the answer!

  Lasse V. Karlsen : You can return a collection, or use yield return, look up the yield keyword on msdn if you're not familiar with it.

  Will : Good point on the yield... I usually don't do that in these types of cases due to a step along the way failing, in which case I just return the empty collection. Its pretty obvious what I'm doing, whereas a yield sometimes isn't clear what happens in these cases for people who come behind me...
  From Lasse V. Karlsen

• Here's the obligatory method form.

  return parents
    .SelectMany(p => p.Children);
  

  And for two levels:

  return oldies
    .SelectMany(grand => grand.Children)
    .SelectMany(parent => parent.Children);
  

  From David B