I'll give you a few more reasons why I don't think it was planned, Dragon.
For one, after Big D and Lok work together getting rid of a potential rival; they are shown living it up all over town. Especially the scene with the two of them in the limo smoking cigars. They were laughing and having a mutual admiration society going on.
The scene that really backs up my point however, is when the two of them are having a talk on a rooftop. Big D is standing right near the edge and the way the camera moves, you almost think Lok is going to just push him right off. He DOESN'T however, and they continue to develop a friendship- or at very least a working relationship.
Here's my point in a nutshell. Why would Lok (wearing very nice expensive clothing by the way) take his kid along fishing in a planned brutal hit on Big D? Not to mention the fact that he had to break out in a full sprint to hunt down D's wife and savagely take her down with a shovel and a branch. Too many things could have gone wrong. Why not a simple shove off the rooftop or a regular triad family hit? In response to JohnR's post: I feel that he MAY have been ruthless enough to bring his son along- but was it practical? I don't think so. Remember, this guy (LOK) is the LEADER of the triad society. I think he would have had unlimited resources behind him were he planning a real "elimination".
As mentioned before- I think that he just finally snapped when he found out about D's desire for shared leadership.
I also saw your comment about "Mob Sister" being not the greatest. That's a shame as I have that movie as well as Election 2 ordered and being sent to me.