Answer: the concrete oprational stage
Explanation: According to Piaget, this stage goes from approximately 7 yrs old to 11 yrs old and allows the child to overcome their egocentric thinking. This means that they can now understand reversibility of things, for example that a litre of water will be the same independently of the bottle or container; this also allows the child to be able to "put themselves in someone else's position".
This child in particular is now able to understand that her babysitter is also an student due to having acquire this ability.
Congress holds the power to declare war. As a result, the President cannot declare war without their approval.
However, as the Commander in Chief of the armed forces, Presidents have sent troops to battle without an official war declaration (which happened in Vietnam and Korea).