I help my children in learning the Quran, but when I perform supererogatory fasting I am prevented from teaching them, so is it better for me to teach my children or to observe supererogatory fasting?

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 is His slave and Messenger.

We advise you to combine between the two things as much as possible; however, if you are unable to do so, then it is more appropriate to educate your children and teach them the Book of Allah as the Prophet said: "The best of you is he who learns the Quran and teaches it to others." [Al-Bukhari]

So, it is an obligation on the parents to cultivate their children according to good morals and Islamic principles, and multiple beneficiaries are better than one beneficiary from which only one single person benefits.

Allah knows best.