When a young child has a nightmare, especially a recurring one, about dying, or something outside of their range of experience, it could very well be a past life memory. Usually a parent can tell that it's not something the child has been exposed to, and there is usually a coherence and authenticity to the story. For example, if you had Holocaust dreams as a very young child and they were true-to-life, and you had never been exposed to anything about that historical event, a past life memory would be the explanation.
Not all children talk about their nightmares, though. I've encountered adult clients who had dreams or nightmares about specific incidents, seeing themselves as someone else, usually in a dangerous situation, leading up to a death. They always remember these dreams because they were so vivid, and they would wake up with strong emotions about the event. As these clients grew up, they felt that the nightmares or dreams related to present feelings and suspected that they were memories. In interviewing them about their present lives, it seemed that their present behaviors or feelings related to the dream in some way. In that sense, it seemed that the past life material was surfacing in these dreams, and was continuing to play out in the client's present life through their behaviors or fears. That's when I would use the dream as an entry point into the past life material.
I had one client who had a disturbing, recurring nightmare since she was a child: She saw herself as a boy in a bombed out building surrounded by other children. The boy knew he had to protect the other children. The client would wake up very disturbed, knowing something bad had happened. These nightmares began when my client was around nine, and continued for years. When we used that image to go into the past life, she got the full story of what happened: She was in Europe during WWII and the town was being bombed and invaded. As this boy, he was told to hide some children near the ruins of a building. He was caught and shot. He died fearing that the children he was supposed to protect were harmed, and it was his fault. After my client saw that the boy was actually heroic in his efforts, the nightmares stopped. She was able to let go of the guilt she carried into this life from that event. And she could finally have a good night's sleep. This client knew that the dream had to do with a past life, even as a child, but couldn't tell anyone. I think that's typical, too. Often children don't talk about the content of these nightmares.
Take the Holocaust dreams as an example,too: If they were actual past life memories, anything you may have seen in a movie or on TV could trigger your feelings about that lifetime. So, sometimes it's not the present images or exposure that creates the memories; the memories are already there. As you get older, it gets a little more difficult to separate out what's inherent memory and what's over-lay from present experience. The "purest" memories are usually those which come up spontaneously in young children or in the dream state. The younger the dreamer, the more likely they are authentic.