Unicef defines a child associated with an armed group (formerly called a child soldier) as "any person under 18 years of age who is or who has been recruited or used by an armed force or armed group in any capacity, including but not limited to… fighters, cooks, porters, messengers, spies or for sexual purposes."
By December 2017 ISIS had basically collapsed. Begum was still under 18 at that point. Considering the limited time that she could have spent in ISIS controlled territory over the age of 18 it is difficult to see her being guilty of any crime in a court.