LINQ - Joining Lists in a linq Query

Question

Question is How do a return a List of B with all the entities in children of all Parents without resorting to the type of code below, I was thinking u must be able to acheive the same within a single linq query?

Class Parent {
    public Title,
    public children List<B>,
}

data = List<A>

var childLists = from x in x.Parents select x.children;             
List<B> output = new List<B>();

foreach (List<B> b in childLists)
    output.AddRange(b);

Thanks.

Answer 1

List<B> allChildren = x.Parents.SelctMany(p => p.children).ToList()

Answer 2

var output = x.Parents.SelectMany(p => p.children).ToList();

Answer 3

using nesting

from parent in x.Parents 
  from child in parent.Children 
  select child;