If you plan to stay more than a few days in Singapore, you'll probably visit Sentosa, the small 'leisure' island of Singapore. There you can find a few interesting places to visit, beign one of the most famous the "Underwater World" oceanarium.
In the oceanarium you can walk through a 83 meters long transparent tunnel, from one can admire a variety of fishes, coral reef, sharks and eventually a diving human doing some maintenance :)
In addition to the tunnel, there are a few other rooms where some spectacular jellyfishes will amaze you with their movements and vivid colors (not from the jellyfish themselves, but from the well thought lighting effects).
All in all, the Underwater World is not worth more than a couple of hours visit, but your kids will enjoy and also they can learn a bit of biology. A complimentary visit to the Dolphin Lagoon, also in Sentosa island, can make your sunday a fulfilling experience outdoors, if weather permits.