[TrailMixRaisin]

Thanks for your input. I was thinking about getting a nice notebook and write letters into it. This method seemed a bit exclusive. I would like to invite grandparents, uncle, aunts etc. to add to the collection and would not want everybody to read the letters intended for the child. This would mean multiple notebooks and more management. While it would make the individual entry more meaningful (I assume you will think more before writing in a paper notebook) it might prevent me to add to it because my input does not seem meaningful enough.

So maybe "simple" mails that are stored by me or by a trusted third party are the best option. This again allows for contribution by third parties.