This trade is difficult to break in the current environment. We'd need to get tough on people who we have generally not wanted to get tough with because we want to keep them friendly for a lot of reasons. Not least of which is growing Chinese (and European) influence in the nations of coastal Africa. We know very well that some of the people we need are less than savory. Some of the trade we engage in is less than honest. But it's not just the coltan trade that's influencing our behavior here. There are a lot of different and competing strategic considerations at stake.
My own opinion? This is probably going to blow up in our face in the future, and we'll spend the latter half of this century, (or maybe the first half of the next?), attempting to convince a rapidly developing sub-saharan Africa that we're deeply sorry for the past but you can trust us going forward.
Something that surprised me: Disney centered their 2016 film The Legend of Tarzan in this context.
[1] https://en.wikipedia.org/wiki/Atrocities_in_the_Congo_Free_S... [2] https://en.wikipedia.org/wiki/Congo%E2%80%93Arab_War
I think this quickly gets into the details though. How much safety is required and what does it cost? Are there alternative materials that cost less than ethical cobalt? What age restrictions should be put on the labour involved and what will those children do instead (both with their time and to earn money)? Where will the adult workers come from to replace those kids and what training do they need?
My point is it is unreasonable to expect a business to be able to do the kind of oversight that would require the reach of government agencies.
We've tried this before, and failed at it before (with respect to diamonds).
This problem should be tackled, but it is worth thinking about likely unintended consequences of whatever power structures you set up to tackle it. I hear the Belgians have some experience ending slavery in parts of Africa.