Is it permissible to teach the Qur’an to children, despite the presence of "a slight lisp in the letter raa’," and to cooperate with another teacher in doing so, especially if the teacher is non-Arab and her enunciation is incorrect?

The Quran should be taught by someone who recites it well, as children imitate their teacher. As for older children who know the alphabet, a slight lisp will not affect them, and most likely, a teacher whose mother tongue is Arabic is better than others, even with a slight lisp. If the lisp affects some words, another teacher can be sought to correct them.