I had a look at the questions, and I am just as confused as you are.
To summarize: Mr. LivingInWales has an adult son from a prior relationship, who is living independently. The son is British from birth, and nothing to do with the application.
For the applicant, the application asks:
Do you have any children of 18 years old or under?
No
For the applicant's sponsor, the application asks:
Does PARTNER have any other children in the UK that are not applying on this application?
__ Yes (provide a whole bunch of stuff, including name, nationality, date and place/country of birth, gender, relationship status, who the child's other parent is, utility bills of other parent, birth certificates of both of the child's parent, etc.), or
__ No
No seems wrong, but saying yes and having to provide all of that for an adult offspring seems ridiculous.