You can consider getting them from party specialists, Sprinkie Parties. They are relatively new to the scene but I've heard so many great things about what they do. You can get decorations (even personalised ones) from them and all the usual party treats. www.sprinkieparties.com As for kids birthday cakes, I love the cakes from freshbakes.com.sg -- they have fantastic cakes which taste and look awesome.
Not sure what your budget is and how elaborated you would like the balloon decorations to be. You can check out these websites for some simple ones: http://www.partywholesale.com.sg/product/balloons/ or http://www.balloonblasters.com.sg/ or http://www.partyday.com.sg/balloons.html You can also look at this shop: https://www.thecakeshop.com.sg/category/cake-menu/ They offer balloons as well. :)
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