Is it ok for a kid's bike to only have a front brake?
Small children usually do not have a grip strong enough to brake effectively with brake levers on the handle bars.
Neither do they have enough coordination to modulate brakes. This means if they were able to brake the front wheel effectively they would indeed be at risk to go over the bars.
Typically, children's bikes have coaster brakes. These allow to stop by back pedaling. Which is an elegant solution: If the child is capable of accelerating by pedaling, the child is also capable of braking by performing the inverse motion.