Children on Adults-Only Cruise?

Hi everyone! I’m currently on a cruise in Europe that was advertised as adults only. For the first few days I only saw adults, most older than me but a few my age since I’m on the younger end of adulthood. I noticed a family on board though with obviously younger children, three of them maybe around 8, 11, and 13. Other guests have noticed it too, including my own family. Does this happen on 18+ cruises sometimes?