I love this idea! How thoughtful of you to do this!
When I was in middle school, all the 8th graders had to do a project on a family member of their chosing. I chose my great grand mother. I collected photographs, took pictures of her wedding gown, wrote down stories about her life, made a family tree, included her wedding picture and her wedding certificate, and her birth certificate. I still have the report I did beck then because I learned so much.
Perhaps you could hve your grandparents write dpwn some stories about their liives, maybe about how they met, where they were born, their school years, how they got married, etc. If writing is too difficult for them, you could take a tape recorder and have then tell the stories and you can transcribe them later.
The one thing I cherish the most out of the report I did the most is the family tree. It's like meeting my family without them even being here. The one I did starts with my great gramothers parents and ends with me, but you could start with your grandparents wedding and include everyond from there.
About the design, use your own creativity! It doesn't have to look any special way. Work with what you have, and the design will come to you as you gather the materials.
There is nothing quite like preserving family history. I can tell you that yur kids will thank you for doing this for them. I know I cherish the project I did! Good luck with you scrapbook!