You haven't picked the easiest / safest places to visit in this region. I've been to Zimbabwe ages ago, but thought it was now unsafe? To see Namibia you need to be prepared for very long drives. My kids would probably enjoy the scenery, wildlife and sports now (at aged 5 and 8), but it would have been quite tedious with a baby and a 3 year old.
I don't know Mozambique, but have known people who have gone there. Both there and in Namibia (and probably Zimbabwe) you really need to think about what risks you are exposing your kids to. While there is a good chance you'll have no problems, the risks are real and you need to accept whether you are comfortable with them. Malaria for example - I was never happy to put my kids on anti-malarials at this age. For one, I never saw the point since these places could be visited when they were older and had more robust systems (there are plenty of exciting, adventurous but safer places to go). Also, my younger child is very difficult to get medicine into. But, many people don't have those concerns.. Another big issue is that you'll certainly find yourself far from good medical care at times. While you could get evacuation insurance, evacuation also takes time (I know from personal experience), and things can go wrong very quickly with young kids. It's scary having a sick child and know that you have to move them to get help (I live and work in Asia so have some experience with this).
If you really want to go to Africa, I'd go for the places that have better infrastructure. Or, why not somewhere in Asia or the middle east?