No, previous posters are entirely right to question how this is being implemented.
Schools have been criticised in the past for jumping wholeheartedly on faddish and unevidenced bandwagons, being sold snake oil with snappy names. Brain Gym. Learning Styles. Growth Mindset. It is reasonable to ask 'where is the evidence for this?'
I've had a scan of the internet and I can see various trials of 'mindfulness' for various things. Fibromyalgia. Chronic neck pain. Anxiety. Depression. I can find very little evidence for its use in, well, what exactly(?) in 8-9 year old children. The trials are also vague and talk of 'mindfulness-type interventions'. Well what is the school planning to do? A bit of deep-breathing? Eating a sandwich in a particular way? Yoga? Talking about stuff? All these things seem to come up under 'mindfulness'.
It's also right to question who will be running these sessions. Primary teachers? How exactly have they been trained to run mental health interventions in young children and was it just a PowerPoint on INSET day? What if something goes wrong?
The internet seems a bit vague as to whether mindfulness is a skilled intervention which requires a regular course of sessions led by an experienced practitioner, or some simple exercises which can be led by an app on your phone. Which version will the children be getting?
I don't think it is ok to simply say 'trust the school' when it comes to a mental health intervention. This is not a new reading scheme or maths project. This is not what schools are in the business of. Have they been sold something common sense with a fancy name by a rip-off merchant? Or have they got genuine experts coming in who will actually know what they are doing? (No, a teacher who has watched a PowerPoint is not a mental health expert).
Parents have to give informed consent when medical practitioners come in and do checks. Where is the informed consent with this therapeutic intervention?