SkyDog75 is correct, rim brakes, properly made and with good pads can stop a bike every bit as well as discs. Your problems are likely due to poor quality brakes and pads on the kid's bikes, not an inherent design defect. There was a VERY long and somewhat acrimonious thread a few weeks ago debating the merits and demerits of disc brakes on bicycles. A search should locate it and reading it should keep you entertained for a fair while.

As to "locking the rear wheel", coaster brakes were good at that but little else. It's a very poor way to control bike speed.