public class RandomDto
{
    public int Id {get;set;}
    public ChildDto FirstChild {get;set;}
    public ChildDto SecondChild {get;set;}
}

SELECT CASE
    WHEN [dtoRandom].[FirstChild_FK] IS NULL
    THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
END, [t0].[Child_Description]
END, [dtoRandom].[Id]
FROM [Randoms] AS [dtoRandom]
INNER JOIN (
    SELECT [s].*
    FROM [Childs] AS [s]
) AS [t0] ON [dtoRandom].[FirstChild_FK] = [t0].[Id]

.ProjectTo<RandomDto>(null, "FirstChild", "SecondChild")
.ProjectTo<RandomDto>(x=>x.FirstChild, x=>x.SecondChild)