Crikey, I don't envy that question. I would be totally honest with her and explain that some people don't believe it even exists but it is a part of you that you can't see or touch but you can feel it. When you get upset or happy thats your soul. When you make a decision its your soul that guides you to make the right or wrong decision. Its up to you whether you tell her your soul lives forever, or whether some people believe that.
fee, an easy way to explain, may be to tell her when someone dies, the part that goes into the ground or gets cremated is only the shell which is not needed anymore.
the good bit which is the important part goes to heaven with jesus and the angels, ( depends if you believe this obviously) rather like a nut or a bannana, the shell or skin is not needed any more.