UK Visitor Visas are required by many foreigners wishing to enter the UK for short stays. Whether you need a visitor visa depends on your nationality, the purpose of your visit and how long you need to stay.
The UK Visitor Visa category includes a number of specific visitor visas that allow people to holiday, visit family, get married, study a short course, sit the PLAB and many other purposes.
A Child Visitor visa is specifically for those under 18 years to meet family and friends in the UK for up to six months.