If you are already doing Burj Khalifa, the Dubai Aquarium is the natural same-building add-on — buy the combo ticket and do both in one Downtown evening, ending with the fountain show outside. If you are doing Aquaventure at Atlantis, Lost Chambers is usually bundled or cheap to add — check the current bundle before paying for both aquariums on one trip.
The honest sleeper hit is Dubai Safari Park: at roughly AED 50 ($14) it undercuts everything else in this list while offering a well-run modern zoo. Families on a budget should pick it over the aquarium without hesitation — allow half a day, and note the park shuts for the peak summer months, so confirm it is open for your dates.
Skip The Green Planet if your kids are teens — it is a young-child attraction at a grown-up price. See also Dubai safari and desert wildlife, theme parks in Dubai, and the Dubai guide.