I only partially agree. While children are perfectly capable of operating mortars and machine guns, it is practically impossible for them to move them around to take a new position for example. They just lack the physical strength to do so. Self propelled weaponry, like tanks for example, are the perfect tool for a child. I've seen them doing it on TV.
also children are smaller so they can more easily fit into tight comparaments making it possible to design lower profile vehicles for lower costs, better concealment and thus better combat effectiveness
While this makes sense, these prices of equipment are very complicated and may go over a child’s head due to limited intellect. I think it would be more appropriate to provide them with their own class of weapon. Attach whatever machine gun or mortar you wish to the front of a fisher price car or other motorized variant and let them have at it. This also reduces training costs.
You are all making great points, but I think could be applying them better. They're small, can't be trusted with complexity, and can be replenished faster than adults. Therefore would be best suited to running aimlessly towards enemy front lines to draw fire, draining enemy resources and morale