Very rarely will you see a royal child slouching, even at a young age. They're taught early on of the importance of maintaining proper postures. The royal kids are told to stand with their feet shoulder width apart, keep their spine straight, and their chin lifted, parallel with the ground.
Their hands are absolutely never allowed in their pockets, as this is too casual for the appearance they need to give off. The royal kids' feet also need to be completely flat on the floor, with their knees slightly bent. No tip-toeing around for these children.
Posture is just as important while sitting. They need to sit with their back completely straight, and their feet either planted firmly on the ground. The little ladies should sit with their knees together with their legs crossed at their ankles. Should the Queen enter the room, the little boys bow their heads, and the ladies do a small curtsy.
Sounds specific, doesn't it? That's because it is. The children have this instilled in them from a young age, both through formal etiquette training, as well as everyday guidance from their parents. You probably thought you had it hard, and you're probably only expected to pull out the big moves for fancy events and Christmas dinners. For the royal kids, this is everyday life!