The most important measurement for the jacket is height - if your child is larger than the height measurement then the sleeves/body may be a bit short, and they will likely grow out of it within a few months. We generally recommend to go with the size up, especially if you want to get 2 winters out of the jacket.
Pants: Again the most important measurement is height, as this determines the length. The waist of all of our pants is elasticated, so will fit a range of sizes. The Splash Pants are a generous fit as they are designed to fit over the top of your child's usual clothing.
Scroll down for diagrams on how to measure your child.