As children often have a very short attention span I suggest that may be a basic start point - short, interactive meditations and meditational activities.
I have also been researching working with PTSD sufferers and some of those suggestions may also be useful. Again even the basic ones such as solid preparation before commencing - ensuring a quiet safe space where participants are made aware of where the doors are, not sitting them near a window etc. helps settle any excess of nervous energy before commencing. I can recommend the book Trauma-Sensitive Mindfulness by David A Treleaven in this arena.
With best wishes