Answer:
Well you have to tell the child that genetics is the study of how traits(characters) are passed on from parent to offspring. Such traits include height, weight, facial features etc.
Chromosomes are the cells that carry those traits in the body. Human beings usually have 46 chromosomes but during reproduction, the chromosomes are halved to give 23 which is in its haploid state so that each parent gives 23 chromosomes each to get 46 chromosomes which is the standard number of chromosomes for humans and animals.
Haploid means a single set of these chromosomes donated by the parent during reproduction while the 46 chromosomes which is achieved after reproduction is referred to as Diploid which means 2 set of the chromosomes from both parents.