I know it can sometimes be difficult to know who a trouble-maker is in the class, especially if you are new to a school or to a class. I hope you were humble enough to apologize to that child?
My advice is to take chances and let the child go to the bathroom. Give the child a time limit - "You must be back in 6 minutes" and write it on a piece of paper or card with your signature on it, or something reasonable like that, and if he/she is not back within the time given, keep the child back for a few minutes after the lesson to find out what happened and, tell the child that if they want you to trust them next time, they must abide by the rules..
Be friendly, show that you actually care! Don't set yourself up as an "enemy", instead, set yourself up as a caring teacher, a mother or father-figure, if you like, but one that creates boundaries and insists on the boundaries being kept.
Dr Grace Anderson