My mum passed away from cancer on Boxing Day a couple of years ago, two days after flooding caused major havoc around the country. We were flooded on Christmas eve, but managed to pull a load of stuff upstairs to safety before it was ruined. In the weeks after my mum's death, I started going through those boxes and in them I found a small box of letters I'd packed away as a teenager, and forgotten about. One was a letter from my mum on my 16th birthday, and a few more were just little notes I remember finding on my bed when I came home from school from time to time. Obviously they were precious enough then to keep them, but since losing her... well, they are beyond valuation.
If I were to put these in my children's lunch boxes, I wouldn't do it every day so that it became boring, but from time to time, with a personalised message on the back, and maybe one day they'll find them stashed away in a box too, and remember how very much I loved them.