He surely has some balls to be talking about this with someone else and suggest a killing of a 5 year old.
Well yea but it was 5 or so at the time of discussion.
Anyways, either do it right after or don't do it at all I'd say, unless he's looking forward to jail time for being sloppy. Even at the kid's age of 5 he's being stupid by letting it live, the chance of being suspected is considerable. Why would he wait another years taking a risk of being exposed and taking a risk of you changing your mind.
I don't think you'd do it out of curiosity, so I guess there was something in return for your part.