Write a book, send a copy to the Internet Archive, upload the digital version. Leave your kids the ISBN or Archive.org item identifier. Donate $2/GB uploaded if you can afford it.

You could also have the Internet Archive crawl your site to preserve it if the above is too much trouble, with it being accessible through Wayback.

https://help.archive.org/help/how-do-i-make-a-physical-donat...

https://help.archive.org/help/uploading-a-basic-guide/

https://hackernoon.com/the-long-now-of-the-web-inside-the-in...

https://news.ycombinator.com/item?id=46611593