LINQ Concat child lists in a list

var allObjectB = myList.SelectMany(x=>x.Children).ToList();

You can just use SelectMany:

var result = mylist.SelectMany(a => a.Children).ToList();

SelectMany allows you to pass in a function that returns an IEnumerable<T>, and it itself then returns an IEnumerable<T> (unlike Select, which would return IEnumerable<IEnumerable<T>>. The result contains all of those enumerables concatenated together.