Is it ok to aid in teaching basic concepts of First Aid as a Registered Nurse to a small class without any other qualifications (etc. teaching credentials) ?

It depends on your BON. In my state the BON has to approve nurses to teach

I would think as long as you emphasized it is basic and always call 911. However, with that being said, I wouldn't do it. Today's society is absolutely crazy and if someone dies and a kid had a hand in it, you would be liable. When people ask advice, I refer back to their GP.

You have to be credentialed by the organization ie American Heart etc

I would think that teaching even a "small class", if they were members of the public (as opposed to fellow church members, social group acquaintances, relatives, etc.) would require some form of recognized credential. "Basic concepts of first aid" would entail exactly what? Are you referring to what I learned as a "Pioneer Girl" (Christian camp equivalent to a girl scout) along with: how to start a fire using sticks, safety on trails in the deep woods, and other simple outdoor 'survival' skills. I would look to existing First Aid course templates and seek any training you may need to satisfy legal requirements. If you even unwittingly represent yourself, no matter how benign or altruistic your intentions, you may end up with unexpected problems or challenges.