The phrase "...if ye should lead her in a fool's paradise..." means to be happy based on false hope. A paraphrase of this line might be, "if you should cause Juliet to fall in love with you falsely..." The nurse is making sure Romeo's intentions are genuine and that he is not leading her on. She has been sent by Juliet to see if Romeo intends to marry her. This line serves as the nurse's warning that he had better be sincere in his intentions, and admonsishes him that, if he is playing with her emotions, that is the worst type of scoundrel as she is an innocent girl. She doesn't share Juliet's message before hearing from Romeo himself that he loves Juliet and plans to marry her.