I actually had ours custom built. Dan is our carpenter and a handyman. He's done lots of things for us for our kids room and for several friends. Knowledgeable. Skilled. Communication is not a problem at all. 9009 0803
If you have a girl, I really like the furniture from Ni-night. They have a store in joochiat. http://www.ni-night.com/furniture/study/ Otherwise IKEA is always reliable!
Not sure if you have boys or girls, but you can look at ibenma for ergonomic desks for kids http://www.ibenma.com.sg/
Take a look at kidshaven. They can customize too and I think their pricing are quite reasonable
Fullhouse furniture sells kids furnitures too. They are located in aljunied.
Piccolo house has really cute kids room furnitures