The have to be properly supported to the jump. This means leg encouraging them forward and straight, but not clamped on, and pushing forward to your hand, but being given their head for the jump so that they don't have contact over the jump and are not punished for jumping. The rider can also sit a little behind the motion of the horse to the jump, so that they have more of a driving seat, and less of a chance of falling off if the horse refuses/runs out/spooks
Put a jump a few meters away from a hedge to slow them down
Make your jumps look scary. Hang tarps on the standards. Ballons. Anything you can imagine.