That's exactly the problem - even if the kids AND the parents are willing, the wider society makes it such an enormous hassle and risk. And there's no guarantee you'd win a legal case, especially if you can't afford to.
[1]https://www.usatoday.com/story/opinion/voices/2018/09/05/mom...
[2]https://www.cnn.com/2015/04/13/living/feat-maryland-free-ran...
[3]https://www.theatlantic.com/national/archive/2014/07/arreste...
[4]https://www.cbsnews.com/news/texas-mom-accused-of-leaving-ki...