To burst someone’s bubble

To do or say something that shows that what they want to happen will not happen, or that something they believe is not true.
Its commonest use is to give news that will disappoint the hearer.

e.g. my son was very excited about going to play at his friend’s house tomorrow. I’m dreading telling him that it has been cancelled because his friend has chickenpox; I don’t want to burst his bubble.

The phrase probably comes from the excitement children get in seeing how long they can keep a bubble intact and the disappointment when it bursts.