Presumably your wife's adult children and their offspring are leading their own lives in Thailand. If that's so, and as you rightly think, section 4 of the FLR(M) form is only concerned with dependent children who are also to be included in the FLR(M) application as part of a UK settlement route. Section 4 - Dependent Children does not apply.
According to the April 2017 version of the Home Office guidance for FLR(M):
"“Child” means a dependent child of the applicant or the applicant’s partner who is:
- under the age of 18 years, or who was under the age of 18 years when they were first granted entry under this route;
- applying for leave as a dependant (sic) of the applicant or the applicant’s partner, or is in the UK with leave as their dependant (sic);
- not a British Citizen or settled in the UK; and
- not an EEA national with a right to be admitted to or reside in the UK under the Immigration (EEA) Regulations 2016."
Wearing my educator's hat, I do wish that the Home Office demonstrated consistency in spelling dependent/dependant!