I think 12 is fine, I have my nephews doing the basic lifts and they're 8 and 9.
In my opinion the key thing to remember as a coach, and this goes for any age, is that you're there to teach them them how to perform lifts, good form, body awareness and technique. You don't teach strong, as their bodies develop that will come with time.
It's not massively different from what I would do if I were teaching someone the Squat, Bench and Deadlift. Form first, if something looks shaky then pull them back. Practice, practice, practice untill it's smooth and then and only then is it a good idea to push the weight up. Of course with kids you need to be a little more delicate and attentive, as they will fatigue quicker and their concentration will waver.