I would get a generic Christmas card and just say Merry Christmas or Happy Holidays and sign it. You can thank her for coming in person and do not say anything about regret or saying goodbye. You can say goodbye when she leaves, i.e., going home. You do not want to bring up relationship issues during the holiday because this is really about your children and Christmas right now. Leave the personal issues going on at the door for the holiday and they can be picked up again after Christmas. If she brings up issues about the relationship, table them until after the holiday.

Again, this visit is all about the kids and what they wanted this year. Celebrate the season w/them, be thankful she's coming at all and just enjoy the holiday for the short time it is around.

Keep it light and simple and no relationship talks!

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.