Your child at this age can understand almost all kind of emotions now - happy or sad, fear, feeling left out/rejected or disappointed. Having these feelings are healthy, but they should know how to cope with it and manage it.
What you need to know
Disappointment is one of the common feelings your child can experience – disappointment over not getting what they want, for not being allowed late nights, for not being allowed to watch TV or play games above the speculated time, not being allowed to go out on their own with friends without elders’ supervision, not being invited for a friend’s party, not getting selected for a sport or event in school etc. Though these seem trivial for parents, for a 7-8 year old it is a major setback and upsets them. They might lose interest, or become sullen.
When they are disappointed, do not try to prove a point by saying “I told you so”; instead listen to them, or if they are reluctant to talk, let them know you are there whenever they want to talk. Do not try to coax them too much to speak. Show them that these things happen and relate a similar incident and how it was managed. They should understand that it can happen to anybody, and it can also be overcome and something good can come out of it. Also encourage positive thinking on how to deal with such situations in future. Motivate them by showing what they are good at and how they can approach the matter in another way.