You are going to have a great time! We went the summer of 2006 with our two girls, 7 and 9.
Of course, we did Mt Rushmore and had a great day. Make sure you do ranger led tours/talks, they are free and make you understand the area so much more. If you have kids, let them do the Junior Ranger program -- it is usually free or a few bucks and they get a badge after completing the age appropriate activities. At the base of Mt Rushmore there is a small cave with a crack/crevice -- you can see part of Mt Rushmore through that crack, a different perspective!
Custer State Park is very cool and it is amazing to see all the bison/buffalo roaming. They also do a very nice chuckwagon dinner.
We also love seeing caves and the Black Hills area has two national park caves -- Jewel Cave and Wind Cave.
We stayed in the Hot Springs area, south of Mt Rushmore and it had a very cool attraction -- Mammoth Site. A new house was being built and when they started digging they bumped into a mammoth bone, and then more. They built a huge building around this sink hole were many mammoths fell and died. Today they are still excavating. My girls did the Junior Paleontologist program and had a blast (we had to sign them up before we left)
I assume when you are in Wall you are planning to see the Badlands? They were awesome. But that was the only part of our trip were the heat got to us. Do this area first thing in the morning before the heat gets too bad.
The pictures you take will be amazing! All of my Mt Rushmore looked professional with the super blue sky and natural light. And you get animals with the Bison and donkeys at Custer. And the Badlands just have to be seen to understand. We also hit Devil's Tower -- great pictures. And I picked up a postcard in the gift shop with the Legend of Devil's Tower on it -- it was great for my scrap page. Remember to take pictures by all the signs -- they make great headings for your pages!
Enjoy your trip!