Based on the information you provided the unit CANNOT require any Soldier to expend funds especially for an organizational day.
The unit can have Soldiers make the cookies if it is during normal duty hours and the UNIT PROVIDES the materials; otherwise this is both illegal and unethical. It is possible to buy the required ingredients out of unit funds. The goal here would be to find a legal and ethical way to achieve the desired goal. Work with senior leadership to find a solution rather than a this is a NOGO you cant make me do it….after all ORG DAY is about team work.