When my youngest daughter started riding on two wheels I added a front brake to her little coaster equipped bike and we practiced panic stops repeatedly as my deal was that when she could stop her bike quickly and on cue from me we could take longer rides and to this day, when I say STOP both my daughters will automatically hammer on those brakes.
So when my littlest was all of 7 she would race down the front walk and hammer that front brake and shift her weight back and at times would cause the front wheel to come to a skidding stop. She practiced this over and over and over so she could show me that she knew how to use her front brake.
Tektro levers and Kool Stops... with two finger braking action.
Neither of my daughters works that rear brake that hard as they have learned that proper stopping technique comes off the front of the bike and not the rear and they have worked at developing strong riding skills.
One of the best upgrades for my daughter's bikes has been the addition of Kool Stop pads and have also installed better quality levers and brake calipers... the cheaper pressed steel v brakes that come on many kids bikes are inadequate so my daughter's mountain bikes run upgraded and better quality V brakes and Kool Stops.
You just can't put a kid on a bike and expect them to figure things out... we'd do drills in empty parking lots and practice everything from panic stops to low speed handling, to laying down mad rear wheel skids.