Adults and children: Someone suffering from Post Traumatic Stress for example may find it very difficult to sit still because it allows them to focus on feelings they have been trying to avoid. This can intensity the feelings of stress and anxiety in the body. There are other reasons why mindfulness may not be suitable which is why it's very important to train with someone who has knowledge of mental health issues if delivering an MBCT course. That way they can offer instruction on how to go about mindfulness in the correct way.
Whilst I personally would not recommend mindfulness for any individual who has recently experienced a severe trauma or who are psychotic, recent studies show that mindfulness practice helps children who are suffering from trauma related symptoms to connect with positive emotional and social experiences, often things that a traumatised brain struggles to do.
The practices in the classroom will stimulate the dorsomedial prefrontal cortex, the part of the brain linked to reflective awareness. I am professionally trained in dealing with trauma and trained in the techniques used by Bessel van der Kolk who is the President of the International Society for Traumatic Stress Studies (of which I am also a member) and he also recommends mindfulness for trauma. One of the studies can be found here.
I welcome any comments.