This may be totally a given in your situation, but I just want to put out there that you want to consider if they even want to be in the wedding. I have to say, I am always made uncomfortable by ceremonial aspects that include step-kids (blending ceremonies, promise necklaces, etc) I realize that this is /me/ being uncomfortable, and it may be a wonderful thing for someone else's situation, but I'm just very aware of the fact that it was the adults who chose to get married. The kids may be anywhere from extremely unhappy, conflicted, feeling that they have to act happy when in reality they are scared or stressed. Any of the above.
I know for us, our wedding was the low point of my relationship with DSS. He really began to breakdown after our engagement and had a very difficult time the day of the wedding. It's been a steady improvement since then (very quick improvement actually, once he saw that his relationship with his dad was very much still intact).
However, the experience has just left me very empathtic to the mixed feelings kids may have about their parent's weddings.