When I was starting this with my dd, I started out with something like:
Native Americans were the first people to find the land we call North and South America.
I showed her on a globe the route they took and showed her how they spread out, with some staying in the colder areas, some moving into warmer areas, some staying along the coast and others moving inland. Some made it all the way to the East Coast!
Then I said that because they moved so far apart, they became very different from each other, and it took a long time for all the known differences to grow up into full cultures (language, clothing, religion, social structure, housing, food and nutrition, etc.). We looked at library books with lots and lots of pictures, and I pulled out some National Geographics that showed Athabaskan, Aleuts, ancient Incas and modern Dine people. She really connected to the pictures (she's that kind of learner), esp. ones of children. Using the web you could easily find loads of interactive stuff for them that would be great as follow up.
I sort of wrapped up the 'lesson' by looking through pictures of modern native americans at work (on the web) - they wear blue jeans, eat sandwiches, drive cars, go to banks and wait on bus lines. It was important to me for her to integrate that this was no different from learning about Jewish Americans, Irish Americans, or other groups that have become integrated into US society. She totally got this at about 3.5 yrs old.