Kids don't gain perspective by being told that they are overreacting. That is how kids learn whether or not their emotional state matters to others.
Kids learn perspective thru the process of being given the opportunity to feel their feelings.
As a child develops, they are able to feel in their bodies the difference between small and big grief which helps them to notice that some things are smaller than others in emotional terms.
When a child is crying over any disappointment, they need you to help them understand their pain and grief so that they can learn how to surf it. Dismissing it gives them no tools. AND it sends the message that their pain is not worthy of attention, which leaves them ill equipped when bigger grief comes later on in life.
When your kid feels grief over something "small", remember your job is to teach them how to feel their feelings and to show them that relationships are a safe place to feel them. The ability to process perspective will come as they grow.