I think that science museums being places for kids is a good thing. The are the ones who benefit the most. If you want science for grownups, you have conferences. Also, that it is for kids doesn't make it impossible to enjoy as an adult, especially if it is about things you are unfamiliar with.
Now, if you go to a science museum and think "only a kid can enjoy that". Then the problem is not that it is a place for kids, it is that it is just bad. It is a thing Disney understood very well, its classics may look like they are for kids, but they are actually enjoyable by everyone, and it is a big reason for their success.
As for art museums, the problem is that they are usually just exhibitions, and to be honest, that's boring, especially if you are a kid. That's unlike a science museum where they actually try to teach you science. It is only interesting if you are already well into that kind of art, and most kids aren't (yet?).
History museums are kind of a middle ground as they can do the double duty of teaching history (mostly for kids) and showing off artefacts to people who are already into that (mostly for grownups).