I suspect it’s legal because it was a prevalent practice before laws were made, and nobody wanted to change that so they just made sure the law allowed for what they were already doing. For example, Kansas was part of a “territory” before it was a state, so only the USA as a whole had authority, and I doubt there were many laws about the subject we’re discussing, so there was probably literally no local law enforcement in most of Kansas at some point. Additionally, in the USA people settled / farm(ed) by having one family live far away from everyone else (as opposed to in Europe where it was supposedly more common to have a central place where a lot of people lived and then they would walk to their sector of farmland that surrounds the town). This means that people with 8 kids probably assigned any 14 or 16 or 12 year olds to watch the younger kids, while the adults and older children were out farming or preparing food (which took a lot more time in the past), since there was no other adult that was physically close by (and any that were would probably have to attend to their own farm/business rather than help with raising someone else’s kids).