What are the safest brakes for a children’s bike?

Children can not interrupt a started movement quickly enough: If they kick ahead, retraction is an opposite movement. In case of emergency the hands react faster! This results in a “task distribution”: the feet are there for locomotion and the hands are for steering, calling and braking!
On our bikes there are child restraints (Woombikes and Frogbikes), which are all adjustable to children’s size. Make sure that the handles are not too big, small children’s hands can be more difficult with the brake levers. If the handles are small, your child has more grip on the handlebars and he is better at the brake levers. Woom and Frogbikes have the smallest handles.