I teach yoga to children, have gone through the Kundalini teacher training and am a credentialed Elementary School teacher.
My teachers, the Gurudass´s advised me NOT to do regular Kundalini classes with children under 13 years old. Kids don´t need to do exercises that move their kundalini in a very powerful way.
I use Kundalini mantras and many poses (Fly Like a Butterfly is an excellent resource, as are the materials from Gurudass Kaur´s Child´s Play workshops)from Kundalini but kids need more variety. We also do lots of dancing. I come up with ¨celestial communications¨", movements to our favorite mantras. The kids love them. And it´s amazing to hear them singing Ek Ong Kar or Adi Shakti or any of the mantras we use in class.
My classes especially love Girish´s Shiva Machine cd, though they aren´t Kundalini mantras, my yoginis love it.
I organize my kiddie yoga classes similar to Kundalini classes, we tune in with Ong Namo and warm up, then exercises and dancing, a relaxation and then a meditation to finish. I have a class that asks to do up to 5 minutes of Sa Ta Na Ma regularly.
But back to the main question, kids under 13 years old, from what I understand, don´t need to do breath of fire or any of the bandhas (locks) that work towards powerfully moving the kundalini energy up.
Hope that helps.