The per-hour charges are the same at all of the Disney hotel clubs, but they recently went up from $8 to $10. Still a great deal as far as I'm concerned. When calculating the cost of taking them with you vs. the cost of the club; remember that they are fed at the club; that means that the effective cost of the center itself is going to be lowered by around $6-10 per child.
As of 3 weeks ago, the minimum increment is 15 minutes. There is a charge for no-shows; that might be the X-hr. charge that people are thinking of. I do believe that there may be a maximum stay; they don't want people leaving their children there for really long stays. Also, you pay when you arrive to pick the kids up; they don't charge for the time it takes you to pry them away from the club.
Depending on which you choose, you may need to time your dinner with theirs. At the NLC, dinner is a children's buffet, and I think that children have to be there by 6:45 in order to eat. On our most recent stay, we had early ressies at Narcoosee's at 6:35, and we were finished with dinner, sans dessert, by 7:30.
(That meal was REALLY delicious, BTW.) It was our last free night, so we picked up DS and took him to Kona for a special dessert with us; the ladies at NLC said that lots of parents do that. It was his second visit that week, we had also taken him on our first night, when Dad & I had dinner at Kona and then went to e-night.