The applicant for the FSP Visa must meet earnings requirements, starting at £18,600, and increasing for each dependent child.
A Family Reunion visa may be appropriate, (Settlement refugee, 2016) and no fee is payable. A Family Reunion visa applies to children, or to actual spouses (people who are married or or civil partners) of someone from whom they were separated when he or she was forced to leave the country, and who has been granted asylum status or 5 years humanitarian protection, but not citizenship.
[The financial requirements suggest the language test, which is A1 CEFR according to Apply to join your family living permanently in the UK, would be the least of the obstacles to be overcome. But that's a consideration extraneous to valdity (?)]
Apply to join family living permanently in the UK (n.d.). Retrieved February 25, 2016, from https://www.gov.uk/join-family-in-uk/overview
Settlement refugee or humanitarian protection - family-reunion (n.d.). Retrieved 25 February, 2016, from https://www.gov.uk/settlement-refugee-or-humanitarian-protection/family-reunion
(2016). Cost of Living in Sudan. Retrieved 25 February, 2016, from http://www.numbeo.com/cost-of-living/country_result.jsp?country=Sudan