Pen/ink/paper combination for archival letter writing?

The birth is being expected by me of my first couple of kids (twins, if you had been wondering just just just how that has been working) come early july. Something them a letter on their birthday, to be compiled and delivered all together at some point in the future (18th b-day that I would like to start doing is writing each of? graduation? wedding?). The point should be to share using them my thoughts/advice/etc to their growing up. Ideally interesting me thinking about my end goal in parenting for them in the end, but certainly helpful for keeping.

So that the real question is. just just just what pen/ink/paper combination could you suggest? The important thing listed here is longevity and design. The letters (paper and the ink about it) have to last 20+ years, even longer in the event that young young ones think such a thing of those. The writing has to be pleasant, too, since there would be significant investment that is emotional.

I am wide ready to accept suggested statements on this, lacking any genuine understanding of just how all of this would interact.