You cannot lie. Spend five minutes and write a note on his door explaining the situation, that you’ve backed out and accidentally hit his / her shrub and that you will speak of the matter in the evening when both you and him / her are present to discuss this. ((That’s what I’d do if I’m given the chance to do so, and actually this happened to me IRL, lol, minus the grave and bunnies…)).
Nevertheless, once he gets home, explain to him the matter and take whatever he says to you like a man. First, offer them inside. Deal with the money talk later. Get the kid something to drink and apologize to them. They’d probably still be hot at you (ie, berate about the dead kitten, bunnies, etc), so explain that it was an accident, that you’re sorry it’s happened, and that you’re contrite (or for God’s sake appear contrite). Ask if there’s anything you can do right now to help. Offer to give the bunnies a proper burial, this will probably console the kid a little, and that the cat is still okay. Now offer to the adult (father? Mother?) to pay for the bush and replant one in. Apologize again, listen to them, then send them off.
Of course, the kid will probably want the problem remedied then and there. Therefore, set him in front of the Tv while you bury the rabbits. The father figure will probably follow you out, so use that time to speak to him as an adult. Mistakes happen, you’re a victim of circumstance, etc. Have a little funeral for the bunnies and explain that they’re in heaven having fun with the cat. Let the kid say a few words - psychologically, this is probably the best.
THEN, the next day, replant the shrub. Offer the kid if he wants to help, and speak to him now, asking if he’s okay, etc (ie, you’re one-on-one with him now). He’ll probably do one of three things: 1. not speak 2. Be ticked off 3. Be sad but approachable, maybe go Carl Sagan on you and start asking questions about the universe, bunnies, where we go when we die, etc. Don’t push it if it’s 1, and let the kid cool down if it’s 2. Three is the best of the worst, I suppose. Console him, tell him it’s okay, and what not. Try to get his mind off of it at least, and explain that the animals are as A-OK as they can be.
Lastly, buy some flowers. Offer it to the parent and explain he or she may place them on the graves, maybe allow the kid to do it if he or she thinks it will help. Apologize once more, and again ask if there’s anything you can do. If so, do it within reason (ie, no million dollar cat statue, etc, etc). It should be within reason anyway. After all, you live with reasonable people, right?
And, that’s kind of it.