otMeshDiagRouterInfo::mChildIterator field

A pointer to an iterator to go through the list of children of the router. The pointer is valid only while `otMeshDiagRouterInfo` is valid. It can be used in `otMeshDiagGetNextChildInfo` to iterate through the children of the router. The pointer can be NULL when there was no request to discover children (in `otMeshDiagDiscoverConfig`) or if the router did not provide the list.