E.M.
On the scrapbooking side, you want things that you can only get on the first day. A picture of their faces when they first see the school bus, maybe the bus pulling away with their faces out the window (if they happen to be looking), or if it's possible, a picture of them with their teacher or teachers.
You also will want memento kinds of things, items from their first day that they bring home from school. If they don't bring anything home, maybe you can ask them to draw a picture of their classroom or their teacher.
You might think of some questions to ask them about their first day. What was your favorite thing to do in the classroom? What did you do first at recess? What did you have for lunch? (if they ate school lunch)
There are more things you can do. I saw a scrapbook layout once that show two pictures, side by side. The first one was the first day of school, taken from the back, but the boy was turning to wave good-bye. So you saw his face, backpack, outfit, shoes.
The second picture was the last day of school. He tried to pose exactly the same as the first day, had the same outfit, shoes and backpack. You could see so many differences in the two pictures! It was a great idea.
I hope this information helps you!