The pickle ornament is a German tradition. The tree is decorated on Christmas Eve. On Christmas day, the first child to spot the pickle ornament is the first to get to open a gift.
Anne is putting the pickle on the tree before everyone goes to bed.
It's a very charming tradition. The problem is, I don't know any Germans who actually practice this tradition.
Nonetheless, I own a pickle ornament because a friend bought one for me, so when I saw this tiny doll sized pickle ornament at a Christmas market, I had to buy it!