All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
There is no harm on you in teaching young children the Quran and Sunnah in a mosque even if those who are responsible for the mosque are people of innovation. There is no doubt that it is better that a Sunni Muslim teaches these young children instead of leaving them vulnerable to the people of innovation. Our advice to you is to continue to teach these children the Quran. Rather, also try to give advice to those who are responsible for the mosque and to clarify the authentic Sunnah to them in a wise and gentle manner. Perhaps Allah would save them at your hands and make you a reason for their guidance.
Allah knows best.