What a difficult question to answer! I searched Polish-American dictionaries and Yiddish-American dictionaries and couldn't find this word. I searched on Google and Yahoo and kept getting the message "did you mean render?" Finally, I decided to try a longshot and go right to the source: the author. I found Jane Yolen's official web site and sent her an email requesting a translation for this word. Ms. Yolen was gracious enough to write back to me and explain that a rendar was the head man of the village, comparable to a mayor in an American town.
I hope this is helpful to you. Please visit Ms. Yolen's page and drop her a note of thanks if it is.