Even though in western societies the marriage of a 12 yo, 10 yo or whatever could have been legal, it was nevertheless atypical. People usually married much later. The kids most likely to be married off at 10 were the children of the nobility and royalty, to cement alliances (and the marriage typically wasn’t consumated early).
For instance, a study done on marriages in France around the Renaissance (sorry, I don’t remember the exact period) showed that women were most commonly married in their late teens to men in their early 20s. A “typical” first marriage would have been between a 25 yo man and a 19 yo woman. Which isn’t much different from modern norms.
Sure, they were atypical. But “not very common” is a pretty far cry from “crime requiring prison time and lifetime registration with the government,” as we have today (at least here - do sex offender registries exist in France?)
Do you want to make a difference for four child marriage? For a fifteen year old girl, forced to leave her family, drop out of school, and get raped and abused untill she is pregnant? Then educate yourself and donate to Plan International. http://becauseiamagirl.ca/child-marriage
If you start from the premise that everyone will act selfishly, then realize that the best “selfish” thing to do is to get others to agree to not do the things that might harm you, the rest sort of falls into place. Adults don’t have to worry about what happens to children because they have passed that stage. Desire for genetic offspring survival could lead to some of those agreements but not everyone is willing to think about their legacy over immediate gratification.
Regarding child labor - it is complicate but it isn’t as if you not buying those items is going to result in the child sitting at home playing video games. Unless the overall economic conditions are changed, refusing to buy those items means the child starves (obviously only if enough people join the boycott).