We are having a bday parties for our girls where to get good balloon decor and kids bday cake. They are 3 and 5yrs old.

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. :)