I have a 10 year old rat terrier and a rabbit. How do I get my dog to not attack the bunny?

The dog wont run after the bunny but stalks it and if the bunny gets close the dog snaps at it and scares the #$#@#(*@# out of it. I have to make sure the bunny room has a gate up and sometimes the kids take the gate down and I'm worried they will come back to a mangled bunny!