I agree with Lemongrove - give huge congratulations and say how happy and proud you are and all your wishes for the next steps in their lives. Write a card saying all this. But I wouldn't give money - the important thing is that they are rewarded for their work by the outcome. They worked hard and they achieved a good result! Its also important, I think, to know that you are not valued because of your exam results, but just for who you are. Even those who pass exams with flying colours will one day fail at something, so it matters that they know they are OK quite apart from exams and the other things our society regards as marks of success.