My DH works with a couple of Syrian dentists and they are equally
about the ages of these alleged "children". Nobody should be allowed to abuse a system which has been put in place to admit vulnerable children, whatever their reasons are. No adult is more deserving than a genuine child refugee in my opinion.
They all broadly agree with the BDA that dentists can't go round X-raying patients without informed consent but there are ways of incorporating X-rays into immigration procedure. It would be expected that most of these refugees would want to receive NHS dental treatment and it is likely that a dentist would want to take X-rays as part of the treatment. Not routinely but if there was reason to do which it is likely there would be - particularly as none if the refugees are presumably going to be turning up with a full set of notes from another recognised practitioner.
If not, both my ex and I had to have a chest X-ray before being granted visas for Australia 15 years ago. We were quite entitled to decline but wouldn't have got the visas
In any event, there's no need to X-ray in the first instance. Just gave a quick check for wisdom teeth. If someone has wisdom teeth, the likelihood is that they are over 17. It's not an absolute determiner but is pretty indicative.