Just add a front brake for safety purposes. Coaster brakes can be fun. I remember using it to drift around corners at some insane speed (for me...lol) as a kid back in the day. But nowadays freewheeling and hand operated brakes is the style so you can do tricks and other stuff.
For a child just learning how to ride a bike, coaster brakes are good enough and can arguably be engaged faster and more natually than hand operated brakes. The learning curve is usually shorter too. It also depends on how coordinated a child is, their hand strength, and if their fingers can grasp a brake lever without their hands slipping off the handlebar grips, esp during braking...not a very pretty sight when that happens and may require a trip to the dentist and/or doctor. Only you know what your child is capable of. Personally, I rather not rush them into things even if I know it's going to cost me some cash in the future to upgrade their bikes or buy them a new one. The idea is for them to have fun riding their bikes, build up their confidence, and using a brake system they can get comfortable with fairly quickly.