E-mail

When your kids are born, reserve them an email address.
Over the years send them photos, notes, kid-isms, stories, news about their achievements,  scan/pic of their art work (this can include the cute hand painting they do on kinder-garden, or the lovely wall art they presented you one day you left them unwatched for 2 sec with a crayon), etc.
On their 18th B-day give them the password.