I've been a student many many years and as an adult (almost) :) I still am. I find that the truth, although it may not be what I want to hear really helps. I find that instructors have the right, and so do I- to know EXZACTLY what is going on. Just tell them the truth, knowing that the bottom line is- they can enter an event whenever the heck they want, but whether they do well or not, is up to how hard they train and work. Which is where you come in. Tell them nicely EXACTLy how you feel. (If you can't completely go with how you feel, put some facts in) Here are some good ex.
"_____ INSERT NAME HERE_____, I think that you and your horses might want some more training and touching up before you start eventing, which I would like to help you with. Your horses show potential, as do you, but the more you train and prepare, the better you'll do. In the mean time, maybe you could enter some smaller one to get experience and pick out what you need to work on, and then I 'll help you in lessons improve. Then as you do improve, we can go to some bigger shows!"
Just an idea