Assuming every kid should have a raincoat, we could say every kid should have its one CAIÁGUA (the Portuguese expression for “water is falling”). These colourful waterproof children jackets (please interpret the word “waterproof” at a literal level) are light, comfortable and functional. This model (86,8% PVC, 6,6% cotton, 6,6% polyester) has breathing holes under the arms and at the back (in the inside), an adjustment knob at the end of the sleeve and a hyper useful inside pocket for all those things kids just love to loose. I'm sure your kids are certainly not going to get wet anymore.
Note: the older brother is 8 and is wearing a 8 years size Caiágua; the youngest is 6 but he was enjoying the "loose look" provided by its 10y size Caiágua...