As a child who grew up in Nigeria and still lives there, I think you are greatly overexaggerating the state of Africa. I had an excellent life and was not at risk of anything that children in the Western world are also not at risk of. I had everything I needed and most things I wanted, and could not have asked for a better country to grow up in. I realise that not every child has this in Africa, but neither does every child in the Western world, so I just thought I should point that out in response to your generalisations.
I also consider The Dark Continent a bit of an offensive term.
Nevertheless, these celebrities are not raising the children in, for example, Chinese cultures as you mentioned. The US and the UK are both multicultural societies so it's not like the child is the only person of his/ her race around. In addition, Madonna and Angelina appear to regularly visit the origin of their children, keeping them in touch with things back home.
All in all, they have definitely improved the lifestyles of their children and should keep doing the little they can to help (as long as they don't keep adopting babies that actually have family, leading to more commotion in the adoption process)