The situation with Margaret can't be judged by UK/Western standards. The Nigerian police are very experienced in dealing with these scenarios, which they've been pretty successful at so far. The father is probably unable to swap with Margaret because the Delta is a thick tangled mass of tropical swamps and waterways and a guide is needed to find your way around. He would also likely be taken hostage in addition to Margaret, not instead of.
The father doesn't work for a big oil company, just a service company but in any case, there's allegedly a slush fund to cover ransom demands. He also owns a bar in Port Harcourt. It's been alleged that the kidnapping is to do with a local turf war and nothing to do with oil.
I hope the wee one is home as soon as possible, poor little girl.