DNA stands for Deoxyribo Nucleic Acid.
Little difficult to explain to a 6 year old, but it is important to answer to their curious questions as it plays important role in their education and growth.
You can tell them that:
It is a string made up of small beads (genes) that decides how you are gonna look. As you see everybody in the world looks different, this is because everyone have a unique DNA. Parents give their DNA designs to their children, so they look similar to them.
In short you can call it Your diagram designed by Almighty!!!
Hope I helped.
Choose any answer but do tell your little one about it, because the future is all about biotechnology and science is going to explore it in depth. And you can also show him/her the picture of the spiral DNA strand.