Before you have to do anything, he will have to file a Motion with the court to get back in front of the judge. I would suggest writing him a letter, certified, return receipt requested (not an e-mail or text, a REAL letter) outlining what's going on and your desire to have him spend time with the kids, but your inability to transport them. That way, you can show the judge that you are NOT trying to keep the kids from their father, but circumstances have changed so the agreement needs to change.
Other than that, he could come to your city and call the police telling them that it is his visitation and you are not allowing the children to come. Then the police will come to your house to get the kids. They cannot require you to find transportation and take the kids. He will have to be there to get them.
There really isn't much to do at this point other than to buy/borrow/rent a car or wait and see if he files anything with the court. But do get that letter off RIGHT AWAY