A child (ie a person under the age of 18) may be able to apply for a visa to join their parent or parents in the UK, whether the parent/parents have limited leave or are settled in the UK (ie they are British citizens, hold indefinite leave to remain or hold leave as a refugee or under humanitarian protection), but the rules about this are sometimes complex.
The applicant will have to show that there will be sufficient funds to maintain them and also that there will be sufficient accommodation for them.
There is no English language requirement for children.