dependent children

When I get married in 4 weeks time and we submit the application for a spouse visa do we have to pay extra fees to include my fiance's 2 children in that application? The children are both under 18.

Unfortunately, yes. The children have to make their own settlement visa applications and pay the appropriate fees. See also sole responsibility.

Not sure those apply here. Are those relevant to children who are being or have been brought up by one parent but the other parent is including them on a spouse visa application? My fiance is a single parent with sole responsibility for his 2 children. They have never lived with their mother.