I have a kids bike with cantis and they are a pain in the rear to adjust. I can not get a solid adjustment to make them easy and functional for him to use.
It is lucking equipped with a coaster brake.
That being said, he at 4 knew how to use the hand brakes and they just didn't work well.
At 5.5 he is almost ready to use gears, but the reasonable choices are slim. I am not willing to spend a small fortune to get a bike he will outgrow in a year.