I have two children - 10 and 13. I have a small bin that I "toss" (paper size) art projects (no big ones), interesting papers - something that represented their writing style or short stories/essays, report cards, invitations to birthday parties and fun cards for the year At the end of the year, every June, I sort through this small bin and save the good stuff in the folder that they used that year the most (the one with the most doodles, phone numbers and obnoxious stickers on it.) I have one for each grade. I have friends who don't keep any of it. But, as a daughter of a "Mother" who did this, every now and then I go through and show my kids some of the stuff that I did at their age. Just an aside...my mom kept a birthday invitation from a little friend's party, in 4th grade, who died the next year. It was quite an event for me back then. Years later, I looked at the invitation and realized she shared the same birthday as my son. I loved learning that. My mom and dad are both gone and I'm grateful for the little stuff that gives my childhood its history.
One thing that my mom did and I try to do too. Make a little notation on some of the papers - something interesting or timely that happened that day, even a weather event. I have papers that are dated with "big snow storm day" or "she came home from school and couldn't wait to show this to me...." or some.....big school story that they shared with me that day. If you are a journal keeper, then you keep these great moments in print, already. This is my outlet for saving these stories for them to read, someday. Good luck!