Anxiousmom-my daughter's ASD was found when she was 1 and repaired via OHS at age 4. It was 13mm with one rim too short for any of the devices. Based on Dr. advice, we didn't tell her until 2 days before. We met with surgeon several weeks in advance, which helped my mental state a bit. He explained that he basically creates and repairs an ASD every time he does any OH procedure to get to the other defects, so he had done the procedure thousands of times.
We spent 1/2 day at the hospital the day before the surgery for blood work, and they were fantastic about explaining everything to her...we have an amazing children's hospital in KC. Surgery was Friday morning. It lasted less than 3 hours from when they took her back. She was only on heart/lung bypass about 10 minutes and didn't need blood transfusion. She was pretty groggy the first 24 hours. Saturday she was mostly angry...like your child, she had no symptoms before. By Sunday, she was walking to playroom on her own and in pretty good spirits. On Monday, we went home, with only ibuprofen for pain. By Tuesday she was asking to ride her bike and play on the play set. Everyone told me that's how it would be, but I didn't believe it. She was back on full activity after 6-8 weeks and check ups have shown her heart is healed and back to normal size.
One thing to consider is they asked if we wanted her to have something to relax her before the surgery. I don't like to take unnecessary meds but decided to do it. She was very calm and sleepy when they took her, which is how I was able to not break down until after I was out of her sight. If she'd been upset, it would've been harder to keep it together. She understands now that her heart had a boo boo and the surgery was necessary to fix it, but she still worries some. Her scar is one thin white line down her chest and doesn't bother her at all. Other kids at this age at least think it's cool.
Having been through it, I don't know that there's anything I can say to put your mind at ease, but I just took the approach that we were very lucky it was found and could be fixed. From reading this site, I know that adults have a much harder time than the kids do. Please reach out to me if you have any questions at all. I'll think healing thoughts for your little one.