We would say this is a too simplistic assumption on your part in forming this question as representing the resolution of prior trauma as a simple matter if one simply has a trusted ally, in the form of a parent in this case, one can rely on and accept their word as truth, and simply let things go because they are told it is no longer relevant. That is not how the mind works, nor how healing comes about. For more trivial things, such an explanation can bring relief, and even for severe things, it can help for a time at least to get some positive reassurance one is not truly in danger, for example, but that will not allay deep fears that will recur again and again and again, which is the nature of how the mind works, and the Law of Karma as well, to keep issues coming back to the front burner for attention over and over regardless of what one wants consciously or what one is coached to do by a parent or caregiver.
There is a self-healing here that is possible and children do much of this on their own spontaneously. So in cases with mild trauma, it is possible that with some reassurance and with some discussion about what happened, the child might be able to work on this, perhaps during the dream state, to replay the events in question and resolve them with a new scenario and effect a healing through their own native ability to strive to be at an equilibrium emotionally, but that is not the majority of such instances in their usual outcome and consequence. Most of the time, such well-meaning reassurances will be a dual message, not just that the event in question is over and done with, but also the child is engaging in concern about something that is not important, at least to the adult, and will be discouraged from bringing it up because they will not want to disappoint their loved one by returning to it again and again when the adult has already dismissed its relevance from the discussion. So that is not evidence of healing, that the problem seems to go away when the children are silent.