If you can let them Rag a few Rats & Rabbits after a more experienced Dog has finished with them, that would be a good start. After that i would suggest to let them have a go for themselves (Whether it be rats or rabbits).
if you can get hold of someone who is going Ratting then tagging along would be a good idea. keep the dogs leashed for a wee bit so they can watch and see a few rats run. then let them off to have a go, once they see a rat/rabbit run they shouldnt focus on other dogs. its up to you whether to take them out alone or together, whatever works best. but if you want to work