Imi dont worry hun you have done the right thing, I cant believe anyone would leave their child like that!!!
You phoned the police not social, so if, if the parents get funny at all, simply tell them how you were concerned at them being gone so long, that you were worried that they may have been in car accident or something, (which may be the case anyway) so its not like you were grassing them up, just that you were worried in case something had happened to them, and you couldnt get an answer from the kids cause they didnt open the door. They cant begrudge you that, afterall that then looks like its cause you were worried about them (even if it was really because you were concerned about the kids).