I keep my laptop in my lap, but I make sure to work in a well ventilated area. A few degrees can kill sperm, but no true studies have been able confirm permanent effects. I did some research on the matter, myself, when I got my laptop. I don't have kids yet, but I'd like to someday. If nothing else, it's not good for the laptop. It has to be able to vent out the bottom. Otherwise, you risk overheating.
Here's some food for thought, though. Maybe you should be less worried about your laptop and more worried about your cell phone. People have already been talking about how magnetic radiation could potentially cause brain cancer. Well the genitals are much more susceptible to radiation than the brain because of the rapid cell production taking place. There's a much greater chance for mutation. How many of yall carry your cell phones in your pockets?