Your kids aren't tiny babies - at 9, 11 & 12 they're old enough to understand the stuff you tell them. Pray with them in the days coming up on the surgery - your prayers will give them confidence that our God is bigger than any of the stuff we're going through - and to place their hope and trust on His unchanging nature. I love the idea of wirintg each a note to open the day of your surgery. Maybe you can even help them write a prayer that they can use while you're healing.
I also would consider that they're old enough to come see you in the hospital - at least for the older ones. As long as they're prepared and they know what to expect - obviously not the first couple of days - but it you're going to be there for a week I'd think the last few days would be do-able.
As for your mom - I think it's mom-nature to get hysterical about their children - even the grown up ones. I know my mom who is super calm about her own problems, tends to worry way more about her kids - and she's a devout trusting Christian who ca easily lean on the Lord at other times. Pray with her too - direct her efforts towards prayer and not towards your kids. Does she have any friends she can vent to so as not to vent to the children? Make sure your sisters know to "talk her off the ledge" when she begins to get nuts. Could the kids not spend the evenings at their aunt's houses at least some of the time?
Urge friends and family to help keep your kids' routines as normal as possible to avoid grandma's craziness.
As for ways to make the weekend special - have an indoor campout. Let them all campout on the living room floor or in one of the bedrooms. Be with them in the the beginning part of the evening - play camping games or board games, eat the homemade pizza on the floor with a table cloth spread out. Then when it's bed time let them be together without you. They can begin their week of depending on eachother this way - remind them to lean on eachother through the week as they navigate homework and schedules. The older kdis can help the yougner and the younger can be the comic relief (or whatever your youngest's personality allows for). This will be a time in their life that they will recall with a certain amount of fondness becuase they had eachother. When I was their age my mother needed spinal surgery - back then it required 5 weeks in the hospital. My grandmother and older sisters were the "parents". There were five of us and the older ones helped the younger ones. Now, more than 35 years later we have a closeness and interdependence with eachother that others comment about.
This is a chapter in the history book of your family - the kids will rise to the challenge and it will create bonds and stories, and memories that will last into their old age.
Overall it sounds like you've planned well. Just keep leaning on His promises.
