Dependents using SET(M) form

I'm British and my wife is Japanese. My wife is nearing the end of her 2 year Spouse Visa so we are filling out the SET(M) form.
In August are son was born who of course is Dual Japanese and British and all ready has both Passports.
However we can't quite make out wether we should list our son on the form. Of course our son is a dependent of my wife, but then he doesn't need to be included in the application as he is a British Citizen.
I kind of get the impression the form was designed primarily for visa applicants with non British partners. Anyway would love to hear some advise and views as reading around there seems to be a lot of experience and knowledge about visa applications. Probably more so than the officials.