[Spoilers Main] Why did Theon think his and Reek's plan would work?

The plan to kill two other kids and say they were Bran and Rickon? It does work, but only because Bran decided to go on his quest for the Three-Eyed Raven, but Theon had no way of knowing that. Theon thought that Bran and Rickon were going to make for a village. Wouldn't he just look like double a fool if he claimed to have killed Bran and Rickon, only for them to turn up alive and well one village over?