I am a divorced father and a stepfather. Recently I have done some soul searching around the issue of parenthood vs. step-parenthood and how and when it is appropriate for which parent to provide discipline.
I caught my son downloading some pornography on the internet. He's 16 years old. In my book this is not a horrible crime, nor can it be ignored. It was near the end of his week visiting, so I did not make a big stink about it. I did take away his computer privileges for the remainder of his stay (about a day). I called his mom, who lives across the country, and told her what had happened. She is going to do something to punish him. I don't know what exactly.
Now, what would I have done if it had been my stepson who lives with us? I think something similar. He has gotten in trouble for computer use and we have taken away his privileges.
I had to really think through the issues on this and make sure I was being fair, consistent, and acting in the kids' best interests.
My conclusion at the end of it all is that the adults who primarily live with the kids are responsible for the bulk of the shaping of the child's character. It is difficult to establish a realtionship where it is ok to be the bad guy when you see the kids infrequently. More importantly, discipline is not effective when it is inconsistent. SO my advice is enjoy. Don't treat them like visiting royalty, but don't sweat every little transgression either. That's really the job of the custodial parent.
That's my story and I'm sticking to it.