Telling a child this age may damage her. Maybe just not being there one day may be best and have a letter or few letters and video or multiple videos for her support later when she needs it. Having kids this age i know trying to explain something like this can go horribly wrong because they just arent able to process this stuff yet. So have your goodbye with her but dont fully explain what is happening just make her feel your love so that is what she remembers(love and fun memories). Later on when she has questions and can understand, she can go to the video and letters. Also it sounds like you have an ex. Dispite whatever relationship you have with her now, dont involve her with the letters/videos. Leave these things to her with your parents and also make note of them in your will. Purhaps keep them in a deposit box at a bank that she only has access too. Im sorry you are leaving this world early. Enjoy what you have left and by that i mean you little girl.