My granddaughter has just experienced disappointment for the first time how do we comfort her>?

Comfort her but don't overdo it...... It's horribly sad and we want to make it all go away but the truth is disappointment is part of life. If tHE adults in her life keep going on and on about it and make a huge big deal out of it then she will too. But if you give it an appropriate amount of comfort then let it go and move on..... so will she. There will be other opportunties to look forward to. Kids are much more resiliant than we give them credit for.