I believe it is a conversation between a young girl and someone close to her after a near death experience. You can almost tell when she says "They had lights inside their eyes... They had lights inside their eyes..." This refers to them being angelic or at least radiant. When she died that's why she didn't see what the other person is asking, she was gone. Though of course "The closing window and the slamming door" could be a reference to heaven and hell, hell being the slamming door and heaven being the closing window. There is more in the first verse that proves it is a conversation between a young girl and someone else after she almost dies but comes back, like when she says "They moved forward and my heart died" She's talking about the people that were coming for her the "kids that I once knew." Which is people she met in life. Her heart dying is really only a metaphor for like the sinking feeling you occasionally get in your stomach. She felt this because she knew she had so much to come back to and wasn't ready to leave. I mean she obviously had someone that cared enough for her to talk to her after she came back. In the second verse when the questioner is asking "Did they seem too real to you?" He wants to know just how far gone she was. The whole verse is basically the questioner wondering how far gone she was is and the young girl replying with things like "They make me feel I'm falling down..." Which is the girls way of saying they made me just unbearably sad. In her case it's okay she just came back, she just saw the representation of people that she once knew. "They were kids that I once knew..." this line doesn't actually refer to children that have died but to what she see after she dies which is a representation of the kids that she once knew. Knew is in a past tense because she is still getting over the fact that something traumatic happened to her and she died but she came back. Dead hearts are people like her people that have been through a near death experience.