pretty good advice. Search the site for "teaching dogs to ignore chickens"
Step 1 is always the same: Supervise your dog!! If you know exactly where your dog is, then you know he isn't in trouble.
Charity's training is right on, except you don't want to start anywhere near the rabbit. You leave the bunny in his usual hutch and go JUST close enough that the dog notices "oh look, bunny!" Start from there. Then, after the dog is reliable at that distance and focuses on you then move closer. If the dog blows you off and focuses on the bunny, back up a few steps. ALWAYS give treats in the beginning. Huge "OMG I can't believe mom is going to let me eat this!!" treats like hot dogs/raw hamburger. And use it ONLY when training this.
It's also a good idea to train "leave it" in other situations as well. Good for when you drop meds on the floor and puppy wants to grab them. Or the dog wants to chase the cat. Or the chickens. Or.....
That said, not every dog CAN be trusted around small furry bunnies. It's just a fact of life that some have too strong a prey drive. Go back to step 1.
I don't even trust my dogs alone with the bunnies when I'm not there.
ETA: always ALWAYS follow step 1. Chasing is a self-rewarding behavior. It's FUN!! So even one time of your dog being able to run up and scare the bunny in his cage is going to set the training back - it reminds him how fun it is. How long this takes will depend on the trainer, how much prey drive the dog has, and how consistent you are with the training. After 7 months, Singe the pup can be trusted off-leash and supervised around the chickens. He can't be trusted around the bunnies yet, but I haven't worked with him specifically to leave them alone. I just tell him "leave it" if he goes toward their hutch and call him back to me.