This is in fact the easiest type of paddling for children.
It comes naturally to them because they feel very stable in the W kayak, and standing upright offers them the ability to apply more power in their paddle strokes, and probably to better perceive the environment and their kayak’s position in it.
The latter is important for navigation, especially when it comes to tracking, which is particularly difficult for small children. There may also be something in standing that facilitates kids’ spatial perception because it’s the position we humans naturally use for moving on land.
You will often see children who are paddling in the lower positions (sitting or riding) stand up when they feel they need more paddling power and control, such as when they’re taking part in a ‘naval battle’ game with other paddlers, or when they have to paddle upwind or counter current.
Because they are small, children can paddle standing even in tandem.
One of the main reasons children feel confident standing in their W kayaks is because if they happen to lose balance they go back to riding the saddle in the mounted (riding) position, which is the stablest. This is also the reason why paddling and fishing standing in W kayaks is a real option for adults too: If you happen to be standing in another type of kayak and you lose balance the only option left for you is to fall overboard.