Answers so far are scattered. I'd suggest thinking back generally: If the problem had been stated as "publish content that lives forever", then we would need t

Answers so far are scattered. I'd suggest thinking back generally: If the problem had been stated as "publish content that lives forever", then we would need to look at ideas with eternity as the purpose. archive.org, bitcoin embedding, at least semi-successful for-profit publication as a book, Long Now - that kind of thing. But the poster asked for "my great grandchildren". In this case (1) this is short term, (2) this is aimed at an extremely specific set of target people, (3) there is already a structure in place: the family. It can be something as simple as "publish photos and writings as periodical PDFs and distribute and entrust to current family members." If there are not enough current family members for redundancy, then include some family friends. With a notice in each volume that you want these volumes to be communicated to your great grandchildren. Let each one worry about hosting or storage. If they loose theirs, they can easily rebuild their stash from the others. If none of them cares to, that's an answer too, but you can fix that by hiring and paying a trustee to maintain sets in 2-3 of the digital platforms mentioned elsewhere (perhaps including archive.org).