Ask an Immigration Solicitor. Get an Answer ASAP.
I am a solicitor.
When and where were the children born? Are you married to their father? What is your nationality?
When were their born?
You may apply for them to be registered as British Citizens if they were born before July 2006: https://www.gov.uk/government/publications/register-as-a-british-citizen-if-you-were-born-before-july-2006-to-a-british-father-form-ukf
All the best