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

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;
}

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;
}

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

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

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

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

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