Combine two child tables based on parent
You may change your code to replace join with left join since you need all the rows from left table, code could be written as:
SELECT        Parent.*, Child_1.*, Child_2.*
FROM          Lead AS Parent
          LEFT JOIN
              Child_1 ON Parent.Child1_Id = Child1.Id 
          LEFT JOIN
              Child_2 ON Parent.Child2_Id = Child_2.Id
;

Left Join - The LEFT JOIN clause allows you to query data from multiple tables. It returns all rows from the left table and the matching rows from the right table. If no matching rows found in the right table, NULL are used.