FWIW, British cycling regulations for bicycles ridden on the road have required a brake on both front and rear wheels on all bicycles, including childrens' bikes, since the early '30s; British law also regards a fixed-wheel/fixed-gear with a lockring as a sufficient rear brake. They still require a front brake, though, and a cyclist was convicted and sentenced to 18 months after killing a pedestrian while riding a brakeless fixed-gear in 2016. Here's the link.