You don't understand much about human beings. Even, Americans who have no immediate British heritage like to visit Britain and Europe. Why, because they want to, they want to feel a commonality with a country/countries of their ancestors. They want to see landmarks and go to key places like Scotland or Ireland.
If a Black British woman wants to take her daughter and husband to experience a part of Africa that she has been to, and felt welcome and enjoyed where is the harm in that?
A 13 yr old already knows what racism is, what it is to be different. Maybe, her mother just wants her not only to have a holiday in an African country that has a lot to offer - Safaris, beaches and the Table Mountain. But, also have a life experience of being in a completely different environment with mixed cultures. It's called 'Exposure' and learning about other countries with a Black majority. Of course, there are other African countries that can offer similar experiences, that are closer, like Kenya and Tanzania. But, all African countries have something to offer the intrepid traveller, as long as they use their common sense.
Her daughter is 13, wonderful age, before losing her to teenage angst. What a lovely thing to do and share with her parents. Her friends are going to be so envious. But, an experience like that will stay with her, as an individual.
As long as they are sensible, book their trips properly and remember that they are abroad, they should be fine.
Your African daughter-in-law may choose not to visit other African countries, that is her choice and probably she's trying to use her limited holiday time to keep ties with her own family/country. It is not the same.