Silence

"I'm ready to take the oath now!" Anthony Burton stepped forward and put his hand on his heart

Robin smiled at him and nodded, he was starting to like this boy...

As for the rest, the situation with them did not much more pleasant, as it is clear that Robin was not joking... 

Even if the oath was symbolic and not really working, this is considered humiliating in the eyes of many of them.

Finally, one of them asked, "And what exactly do we have to swear, if I may ask?"

"Good question and the answer is simple, but listen carefully because many things will depend on it.

You will swear not to leak any non-public information about anything to do with the Burtons in any way, and not to use what you have learned here to do anything without MY direct command, or else the penalty will be death.