Went with my wife and children (10 and 8) during Easter week, so it was busy. We queued for tickets for about 20mins, the queue was actually shorter by lunchtime. Buy a joint ticket for the big wheel if you know you want to go on it - it saves a little money, although they don't do that for kids, you buy separately.
Inside the Aquarium you start the visit on the top floor and walk your way down. That means you see many of the same tanks at the top and the bottom i.e. you see penguins or seals on the land, and then as you progress down the building you see them underwater.
It's far busier at the start of the tour, so for example, when you see the tiger shark (the main exhibit) don't worry about the queues, just keep moving - later on you'll find you can even sit down and see it up close! At the end there is a fish/ray touching pool, which my kids liked too.
A bit pricey, but a good few hours and a some educational value for the kids too. One of the best things to do with kids in Osaka.
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