Thanks man! Personally, if I had to pick which of the two programs was more morally justified I would far and away go with Spartan-III, in large part because I consider Ackerson's rationale for creating and deploying the IIIs to be far more sound than Halsey's arguments for creating the IIs. The IIIs are created when humanity is genuinely facing what is inarguably an extinction-level threat, and are given clear and achievable goals and the tools and support necessary to accomplish those goals, for all that they are still traumatized children being pressed into military service to sustain massive casualties. The IIs don't really have this excuse, or at least have far more holes in their mission and how they are set up to complete it.
I'd recommend that book, but with the caveat that it's far more focused on the history and background of the Congo and the Congo Wars than on child soldiers specifically: while it does talk about battles and combat, it talks far more about conflict minerals, refugee flows, interference by other African states, war crimes and genocide. It's a damned good read, and it dispels a lot of common myths about the Congo and the wars that are being fought there, but it is very, very sad. Make sure you have something suitably uplifting to balance it out.
If you want something more focused on child soldiers specifically, I'd recommend They Fight Like Soldiers, They Die Like Children by Romeo Dallaire, who commanded the UN peacekeeping mission in Rwanda during the genocide, or for a more academic take, Armies Of The Young by David M. Rosen. I haven't personally read it so I can't 100% guarantee its quality, but if you'd prefer something more personal, A Long Way Gone is a memoir of a former child soldier who fought in Sierra Leone, and has come highly recommended to me. None are very happy reads, obviously, but then, it would be pretty tough to make a topic like this amusing without coming across as a sociopath.