Dearest Lars,
I wish I could tell you where I am, but please know that I am safe and well. Try to cheer up your brother and sister the best you can, and tell them I love them.
You must hide the letters you get from me, and never tell anyone about them. I won't ask you to keep any other secrets, but please keep this secret. I hope when you're older, I'll be able to tell you everything.
Until then, I want to hear all about your studies and your games and everything going on.
I love you,
Mother