Say that mommy is storing them in the attic and will take them out when they are old enough to go to college so they can take it with them to the dorms :D
I little lying like I'm "storing" (read throwing) them away doesn't hurt.
put the best pieces in a scrap book and write the dates and what the picture is of (sometimes it hard to tell) if you can take picture of 3d models if you dont want to keep it and add to the scrap book, throw the rest away.
Make up a scrap book for each year, with the little stories the write, pictures, thoughts funny things they've said, pictures theyve taken, etc.