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The manuscript 'Think Python' (http://www.greenteapress.com/thinkpython/thinkpython.html) includes a chapter where they use a Turtle to create simple drawings in Python. This book is aimed at beginners.

Is it something that I can install in a computer so a 6-year-old who never heard about programming could use it?

No, not really. Involves installing Python/IDLE and then importing a module. Not difficult, but probably not what you're looking for!