The saddle on a balance bike should be set so that the child can have their feet flat on the ground. Most decent balance bikes (https://www.littlebigbikes.com/shop/convertible-balance-bike/) have a quick release seat clamp for easy adjustments. They should be able to reach the handlebars comfortably, with their torso angled slightly forwards. Imagine pushing a broken down car, you naturally lean forward so you can brace against the ground. If the torso is too upright, it can be difficult to push forwards.