I've never seen such a young chick be so... lively, let's say.
It'd be difficult for me to take seriously since it's so adorable now (the movements with all that jumping, not the bullying itself), but I can imagine it becoming very bothersome and more dangerous later on.
Perhaps you could observe them and correct the chick? Adults lightly peck the top of the head if they want to correct other birds, so maybe carefully tapping the head of the chick with a finger could work. I assume it would take quite some time and lots of tapping but the sooner you start the better.