You have not provided the ages of the children, but bear in mind that children seated in another cabin are considered "unaccompanied" and their travel will be dealt with as though the adults were not physically on the same aircraft.
Because South Africa has special requirements, you should consider how those will affect, if at all, BA's own requirements.
Bottom line here is that in an emergency or even with turbulence which confines passengers to their seats, having your children in another cabin may pose significant issues.