The next thing is to explain everything that's happening, or walk them through the process and emotions. With someone taking them through everything while in the moment, they will be listening if you have to call it off when the excitement is high.
Right before the shot, it's good to have them dry fire a couple times. Naturally, they'll be super excited and flinch like a SOB if it's the first shot on the first kill of their life.
Every kid that I've taken out has had different levels of rifle experience and shown different levels of excitement, but when walking them through everything they've performed admirably.
It also never hurts to get snuck up on the animal as well for the wow factor.