No more often than a normal child would. Bipolarity is most often a lazy diagnosis and more times than not can't actually be diagnosed in children. Looking for symptoms often just leads to frustration for both parent and child.
Night sweats in not a listed symptom used in the diagnosis of bipolar in children. However disturbed sleep, including nightmares and inability to sleep are common occurrences. Night sweats may be triggered by these events or may be a side effect of prescribed medication. Make a note to discuss this with your health professional during your next visit.