I wish I did have some advice for you.... my 5 year-old daughter goes thru the same thing. She was a nervous wreck when I was hospitalized last year, & is nerved up about
my upcoming surgery also. I don't know how to reassure her- I do tell her what exactly is going to happen, what to expect afterwards, & she knows all about
stomas & bags. She's even seen stoma pics on the internet! And when I do break down & cry over what's going on, I know it bothers her. She is pretty resilient though, & she knows that Mom will be 1000x better after I do have surgery- no more pills, hours in the bathroom, weird infections & such. I also think the younger a child is, the easier it is for them to accept these things- 12 is a tough age anyways, never mind having an ill parent on top of it! Maybe you could call her therapist & ask for some help with it?
Or, another good place is the library- in the youth section, they have tons of books about
parents being ill, divorce, death, homosexualioty, etc.- all the things that are tough to explain but are a part of everyday life. This indeed is an awful part of the disease.