If you are on a budget, you could consider Batam! I know lots of people have the misconception that Batam is for sleazy men with dirty intentions but it's actually quite a fun place to bring your kids.
There's Go-karting you could do, or high net activities like walking across ropes at a high, etc (Barelang Acticities) and the seafood there is pretty affordable.
A return ticket to Batam with taxes only cost $48/per person and the ride takes only about an hour from Singapore.
Read more