Since there wasn't enough character room in the "Other" text box:
How about hand print cut outs of the children's hands (Violet now, and Henry when it's easier to hold his hand still enough to trace) that increase in size over the years.
Just trace the hand print onto a fabric, and cut two. Then fill them with something like cotton and stitch the two together, forming a stuffed hand print that you can hang on the tree, maybe write the date and age ;)
Eventually there will be bigger ones but then maybe you can pass them on for them to keep :)
I heard of this from a nanny I know and I wanted to start next year with Sebastian.