On the Visa application form to the UK it says any children (Thai wife going to UK for holiday with me). Her only son is 18 married and has a daughter, does he have to be mentioned on the visa form. It doesn't say dependent children.

I know this sounds like a stupid question but my first wife (25 years ago) had her first visa turned down for something stupid on the form. and I don't want any problems this time.

I take the view that uk immigration deal in absolutes , so does she have children or not , even if the " kid " is 90 she is still her kid , I would put yes to the question then if they ask anything else you can know that you put in the truth , much harder to wriggle out of it later , and as you know before you were refused for a similar mistake or misunderstanding , I can see your position he is not a dependent child , but unless they said " does she have dependent children " then you have to put yes she has children , only my opinion , maybe a more experienced board member may know better ,

She should enter his details.

Not because she may be refused if she doesn't; how would they know?

But if she says now that she doesn't have any children, this could have a seriously detrimental effect on any future UK visa application her son may make. For example, to visit her should you and she ever move to the UK.