Zoidy, very pleased to hear that you are re-assured by the hospital, surgeon etc.
I have no experience of heart surgery, but my DS did have 4 ops over 5 weeks when he was about 10 months.
The aneasthetist will talk to you before the operation, the day before or whatever. Ask him/her what the procedure will be: I stayed with DS and held him until he was unconscious each time. The aneasthetist is also the person to ask about what post-op pain relief, and how they intend to handle it.
Find out from the ward sister where you should wait while the op is in progress (and at the booking in or assesment meeting, where you sign the consent form,) ask the surgeon roughly how long the op will take. They will call you as soon as your DS is in recovery, so they need to know where to find you - IME there is often confusion arund these sorts of things!
Have someone with you: the wait while your little one is in theatre is unbearable. Work out how yu will inform the relatives who will be anxious at home; you won't be able to use your mobile, and you won't want to spend loads of time on the phone when you could be comforting your baby.
Think about food for you. The canteen is often horrible and miles away from the ward. Have snacks and someone to fetsch sandwiches.
Children's wards, Intensive care and HDU's are emotional places - expect to meet parents with children who are far sicker than yours. A bit like MN, they are also places where parents make intense relationships with each other. While DS was in theatre for 5 hours a woman whose 2 year old had fallen from a window physically clung to my neck waiting to find out if her child would be brain damaged(he wasn't).
And the best thing? It is all over in a flash, it is nowhere near as bad as you will have feared, and please believe me, the kids sail through it while the parents are in ruins!