Carrie,
The bar area at the
Wave is pretty straight forward bar. While the overall bar/restaurant has a great cool and contemporary feel, the bar is that standard rectangle of high stools that surrounds the bar. You don't have booths all the way around the bar area, so I would say that while it has a great ambiance, it is far from ideally suited to a 1 and 3 year old.
I think if you plan on attending with them, expect that you will want to grab a table in the main restaurant area, adjacent to the bar area.
Another choice is to use the Kids Night Out service. This babysitting service is not officially run by Disney, but they list it on their resort web pages in the amenities section. You could let one of their bonded babysitters watch your little ones in the room while you enjoy your sisters big day, or pre-day, as it were!
If you have any follow-up questions, please feel free to ask me directly from my bio page!
Have a magical vacation!