During an U11 travel baseball game: bases were loaded, ground ball hit to SS. The SS fielded the ball and began to throw to the Catcher when he was hit by the runner advancing to 3rd. (unintentional) The SS lost the ball. The umpire called that runner out, but allowed the score from the runner at 3rd advancing home. Was this a correct call? If not what is the correct call.

The ump got half of it right.

Interfernece call is correct. Dead ball runner out, no runner's can advance, unless forced.

Since the runner going to third was called out, the runner on 3rd is no longer forced.