Start an email for her and send pictures and letters to it throughout her life. Then on her 18th birthday give her the email with all the memories.
I also sent many articles about personality development etc which I started to realize some info is too advanced and wish somehow they could be viewed at the "right" times in his life. Like he doesn't really need to know nor understand neuropsychological advances in research :) So little pondering on how to "feed" him the right info at the right time. Also extremely timely, since its email, if I miss a particularly important day sending it out, I'm late :(.
Nevertheless, if gmail holds out for the next 18yrs, it would be a killer present to have his entire life online w/ pics and at the time memories handed to him. and like other commenter suggested, may need to do a regular "takeout" for him.
Furthermore, in addition to that, I'm also trying to address the needs of our immediate family sharing his day to day fun but don't want to use the currently available privacy fiasco social media.
So I'm also in this dilemma like others howto share w/o going to FB/Twit/Insta and alike. Want something very private but easily available. And like the other commenter, I came to the conclusion of creating my own. So I'm on my way of wireframing, prototyping a PWA based service where everything is private and is fee based. Kinda like the antithesis of all social media as we currently know it. Time seems ripe to address the need for privacy....