With kids, it depends on the day.
A remarkably intact, multi-level brick complex of vaulted halls — administrative offices and markets rather than the 'mall' the internet calls it — overlooking the Imperial Forums, with a museum inside and great views down Via dei Fori Imperiali. The real draw is legibility: far quieter than the main Forum and far easier to read, so you actually grasp how Romans used the space instead of staring at anonymous stumps.