For others reading and commenting, here's the situation as I see it and also a couple things I do NOT see, as they have not been mentioned.
The foreign spouse Ukrainian, which is pretty much irrelevant. The father of her children is Namibian. Why they are not also Ukrainian is irrelevant, as they are in Africa, and the petitioner indicated all would apply for visas in S.A. where they live. The children do not have approved I-130s yet, so aren't part of the current issue except being counted in the household size.
To really understand more, we would need to know answers to questions I've asked that have not been answered yet. The OP may not wish to answer those questions in this forum, but it limits our ability to assist and advise.
We need to know exact wording in the denial letter. All that has been quoted are from form instruction. We need verbatim reasons given and exactly what the officer indicated was needed. Since there is no such thing as a co-sponsor, clearly that is not what was asked for. Perhaps they gave the option of a "joint sponsor". That would be a positive, and indicate it's less likely there are other issues besides the financial.