This is a peculiar line. It suggests to me that Claudius isn’t entirely sure what to do anymore. His plans are unraveling and he can’t be as directive as he usually is. He’s hesitating, I think. He doesn’t think Laertes should hit him now? He doesn’t think it, he says. It might not even be a full sentence. I do not think’t – but Laertes is off, already talking to himself, revealing that they are both beginning to question this plan.
If Claudius wanted Laertes to definitely not hit Hamlet, he could say, “No. Abort. Abort.” Or something to that effect. He’s a king; he knows how to give orders. He’s done this sort of thing in public before. He could have them give o’er the game and call for lights. But he just manages to get out, “I do not think’t” – which Laertes either does not hear or interprets to mean “Yes, he should stick Hamlet with a sword now.”