Take them to the Singapore Zoo, older kids love it there! Also there is a water themed park called Escape theme park. The Botanic Gardens has a kids area or the Science museum if they are into that sort of thing. Fidgets is a soft play gym, good when it is rainy outside.
The Luge/Segway/Merlion/Sky Tower at Sentosa... Wild Wild Wet (but make sure you get there from the opening - it's empty but then gets quite busy around 12pm). Science Center for sure and also the National Museum - not exactly a "kid spot" - but my kids love it - they put on the headphones and follow the numbers around the museum - it's never crowded and good cafe at the exit for lunch.