Entity of the structure: the LINQ declaration results in a more efficient SQL query for grandchildren

What is the correct / best LINQ statement to get the most efficient SQL query to get a count of rows of grandchildren from a single parent? Also, is this possible in lambda or does it require query syntax? Details:

1) Main table -> One to many relationship with the secondary table

2) Children's table -> One to many relationship with the grandchildren table

3) Count of rows of grandchildren with a parent ID that may or may not be valid

4) from a DBContext

I guess the query syntax would look something like this (?):

var parentID = 1;
var account = (from gc in context.Grandchild
join c in context.Child in gc.ChildID equals c.ChildID
where p.ParentID == parentID
select gc) .Count ();

If that is correct, what is the lambda equivalent? Thank you