WOW! Betsi, this really is a dilemma - BUT your kids DO need you and the ticket is bought and paid for and as you say, can't be changed. Let me say this.
No surgery is ever completely without risk - BUT, open heart surgery is about as routine as you can get these days. It sounds much scarier than it really is. She looks like a great candidate - she's not sick, in good health, did not have a heart attack, and everything should go very very smoothly! Gary had open heart surgery (and he was in heart failure) and was home in FOUR DAYS and we traveled out of town 4 days later!
He was tired, but he was fine!
Of course, you want to be here with her and if there were different circumstances, if it were an emergency, if she was very ill, if she had few people to care for her, etc etc, THEN I could see you staying by her side, because her needs would be greater than your children's needs.
I know you are torn, I would be too, but I would go to Seattle, and check on her frequently.