I recently went to the EMP Music Museum with my 2 kids ages 3 and 1 and I think overall it was a huge mistake and overpriced for what you actually got to see. Now, it may have been a lot more fun if my childrten were older as there was not a lot they could do. The music lab for instance, its a really cool idea but I didn't get the chance to play much because of the crowds you get on weekends.
So, if you can find a babysitter and go on a weekday it is excellent. If you need to go on a weekend day or with kids I would skip it and go to the science center instead.
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