Go for it. I've got 9, 7 and 5 year olds and they all love it. The 5-year-old started playing when he was 4. There are a few more complicated games that she might struggle to do well at, but she'll still have fun. My 5-year-old has nearly reached "pro" status on a couple of the sports games.
6 is definitely old enough... I mean, she won't play Zelda, etc, probably... but Wii Sports (comes with the system), Wii Play, Carnival Games and many others will be fine for her. Most of the games don't require anything more than pushing one button and moving the controller in a certain motion.