From what I've read, Wind Waker was aimed at all audiences. The graphics style was a new test that was aimed at a younger audience. However, the graphics style was used because of the water. The developers felt that if the water was clear, the people on the islands (hypothetically) would think, "hey! What's that down there?" And it would have been way too hard to make it realistic. Real water changes colors at certain depths, people would wonder why they couldn't see fish, the octorocs and other enemies would be able to be seen, they might have made Link have to dive to retrieve things, and a few other reasons. I feel that it was also used to bring Zelda back to its roots. When I think of the first Zeldas, I imagine a cartooney world where anything is possible. Not a super realistic world like the one I live in right now. (Which is also why I think they chose a similar style for SS and not the more realistic approach that TP took.)
Now, some of the aspects were aimed for an older audience. From what I've read, the puzzles and exploration were aimed toward the older audience. They hoped that the older audience would buy it for the puzzles. But as its turned out, many people think that the puzzles and enemies were way too easy (when I first played it, i actually thought it was pretty hard, but over the years, its gotten easier.) The exploration has always been my favorite part. There is so much to do in that game due to the exploration. Its kinda exciting when you find a new island that you've never visited or never completed.
So, all-in-all, the game was intended for a wider audience than just children. The game is a great game that adults that I know love.