Recently I have read chapters 33 and 34 of the book The Thief Lord. In chapter 33, Mosca, Riccio and Prosper came home to find out that Hornet and Bo were missing, so they decided to go and see Victor because they thought Victor gave away their secret about the rundown theater. In chapter 34, Scipio found out about Hornet when h got home because the police had found out about the theater. Hornet had whispered to Scipio that Bo was with his aunt.
I think that Victor had nothing to do with Hornet's and Bo's disappearance, but I don't know how the police would have found out about the movie theater though. I think you should read Joey's post because I think he did really well describing Ida Spavento's story.