dd had 13 operations between age 18 months and 2.5.
She doesn't remember the hospital, the operations, the doctor, or the problem they were solving.
It wasn't heart and the ops were all day ops, so much smaller, but you would think after 13 times she would remember something.
We had a hospital dolly, (and several other soft toys that were specific to the hospital trips) The dolly was small cheap one from tesco, but crucially it had eyes that closed. I used to talk her through going to hospital, being in a bed, going to sleep, like dolly and waking up in hospital bed with Mummy.
The play therapists are amazing and they will act it out (bandage on hand, dolly getting temperature taken etc) with the child before hand.
Possibly the worst feeling in the world is seeing them go off to the theatre, and then waiting, I won't lie, that is tough, but knowing they are in safe hands helps.
My neice has had 3 major heart ops, and they used to have a complete first aid kits with all the hospital stuff in, like syringes etc and play hospitals a lot afterwards each time, it was her way of processing the experience