He would have literally no reason to kill the Stark kids when/if he caught them. They're way more useful as hostages and he'd have to be pretty damn certain that Robb doesn't come back from his war, because he'd be in very deep shit indeed when he comes to hunt him down afterwards. Same reason Sansa is still breathing and Arya was searched for and iirc to be captured alive.
He would probably not hesitate to kill Osha, though, as she orchestrated the whole escape.
My call is, this is a bluff. Make the peasants (and the Maester) believe their little Lords are dead so they don't step out of line again.
Side note: The whole "We send two orphans to help you" with "Hey we're escaping past that dude we gave those orphans to" is pretty much a Chekov's Gun
. If those buggers have no purpose, why bring them into play and if it's not them being roasted alive, what purpose would they have served? You could argue them being a Red Herring, but combined with it not being exactly the smartest course of action, I doubt that.
Side note 2: If it was the Stark Boys, there would be no reason not to publicly execute them for good measure instead of just hoisting the burnt corpses.