We love Langkawi but always stayed in a very high end resort where they gave us a special rate for Malay/Singapore residents. Find most good places quite pricey, but well worth the money if you want to splurge.
Really worth considering and more affordable are the following. They are kind of back to basics but all small idyllic islands, and full of boho chic singapore expats. garanteed to find good company for the kids. great diving too.
the perenthian Islands (Tioman, Redang). check reviws on tripadvisor.
Divers Paradise and a short flight from Singapore or 3 hour drive into Malaysia ( a great way to see the lush nature, pineapple plantations etc?..). Safe drive with taxi drivers from Singapore (about $350 for an air conditioned mini van - safe drive) + ferry from Mersing.
Sibu? see Rimba resort, not been but heard good things.
There is this very popular little island just 25mn boat ride from Mersing which is super popular with singapre expats. On the tip on my tongue, but cant remember the name!!!!
Try the living overseas forum and fing one of the many singapore threads, someone might help?
you can also fly very easily into phuket or Krabi and go on to so many good resorts. Then catch a fly back into KL.
try jetstar / silk air / bangkok airways / air asia & tiger airways for cheap connections. (you can book one leg only for each trip)
HTH