This way the child will eventually know that the parents never stopped loving them or never stopped wanting to be a part of their lives. I'm thinking if this happened to me I would entrust a close relative with this password so the child could have access before age 18.
This is just a good idea for all parents as a way to save report cards, certificates, poems, etc. I wish I had thought of it myself. Might be a good idea for estranged grandparents as well.