Personally, I'd prefer a more destination-ish location than Indy so I can make it a family vacation either before or after Nats. The highlights of Indy's tourism website don't scream family fun (except the Children's museum) for 6 + 9 year olds. Did anyone go for more than just Pokemon and was there enough to do?
While in Indianapolis, my (8 year old) son played in Juniors, I "watched" him play. My wife and (5 year old) daughter did other stuff. The city had lots to offer, they were active longer than we were at the convention center.
My wife and daughter went to the zoo which has a large butterfly exhibit and enjoyed it. On that weekend Indianapolis had a Native American festival two-three blocks from the convention center, my daughter loved it. At one point they walked in to a drum museum/expo where she pounded on many percussion instruments and had a blast, it was at the other end of the mall from the convention center.
At one point we all went to the river walk (again, walking distance from the convention center) and rented a paddle boat, it was tiring, but the kids enjoyed paddling in to the water fountains and getting wet. There are also family bicycles for rent there, saw many families doing this.
Then, the only place we drove to was the children's museum, it really is good. We travel a lot and make it a habit of going to children's museums, this was by far the biggest out of the twenty or so different ones we have been to.
There is also the NBA hall of fame that is two-three blocks away from the convention center, we were going to go there the following Monday, but alas, it is closed on Monday's.
I am sure there is more to do there, but my wife didn't want to drive and everything they went to, they walked to.
All in all, I vote for a repeat of Indianapolis.