The cadets are allowed to leave for breaks after LMD (last military duty). In December, usually that is their last exam for doolies. This year, exams run from Dec. 17 to 20 (and there is an evening exam block), but that doesn't mean necessarily that your cadet will have exams each day--there is some flexibility. Last year the exam schedule came out around Parents Weekend if I remember correctly. The cadet wing has to return by Thursday, Jan. 3 at 7 p.m. according to the latest academic calendar.
Although I realize prices can change, it may be worthwhile to wait until you know your cadet's schedule unless you can make reservations with an airline that doesn't charge change fees.