A uk fiance visa basically grants you entry in the UK for a duration of six months. During this time, you are required to marry your fiance in the UK.
If you and your fiance have children who are under the age of 18, they are allowed to enter the UK as your dependents only when it can be proven that it would be a problem for them if they are not allowed.
Also fiance is supposed to prove that they can understand and speak the English Language. It is a part of the application that you have passed an English test from an approved test provider.