what can these children do with this new knowledge besides talk or deal with teacher-initiated problems? How can a child actually use it to do something that has real personal importance now?
The principle is called the power principle or "what comes first, using it or 'getting it'?" The natural mode of acquiring most knowledge is through use leading to progressively deepening understanding.
But in any case the solving of an isolated problem just because someone asked you might be fun but it is not what mathematics -- or life -- is about.