Sure, if someone dies you can - or you even have to grieve. This is where most people understand most easily that griefing is the emotion that enables the loved one to let go.
However, if a child has to let go of something, especially if it is something unimportant or a trifle from the evaluation of an adult, then the child should not grieve but function properly.
Given the knowledge that griefing is the emotion that allows you to let go, it is the most sensible solution to grief if I want to or have to let go of something instead of trying to persuade you that it is not bad at all.
Even as a well-behaved adult, I sometimes have moments when I find it difficult to let go of seemingly minor things. If I allow the feeling of grief in such a case, I have often experienced that letting go and accepting what is much easier is easier for me.
I noticed that there can be other feelings before the grief, i.e. before letting go and acceptance. If that is the case, then letting go is only possible when I come to terms with it.
I have had the experience with my children that if I manage to let them be in your grief (however it expresses itself - loud sobs, soft tears, …) and, if necessary, to accompany them with understanding and consolation, such situations run more beneficial for all of us