When you say William
Valentine registered all his children, who were born in the 1830s/40s, what exactly do you mean? Where and with whom did he register them?
The British Consular records from Fiji, which run from 1858 to 1873, do not contain any
Valentine children, and the Fijian colonial government registrations start after
Cession in 1874. Unless they were all registered late, which could happen, these children are unlikely to be in the Birth Registrations.
The children could be, and probably are, in the Marriage registrations, because by then they are old enough to be getting married.
There may be baptismal records, from probably the Wesleyan church.