I have worked as a volunteer within the Scout Association over the past ten years and have filled in more CRB forms that I care to remember. I remember the opposition to it when it was first introduced in our district - not to dissimilar to the comments here.
The fact is that if you work with people who may be described as vulnerable you have to have a disclosure. I don't think there has been any incident to cause this TRAININGCOM to be issued as FOK suggests, it's just a natural progression. It started with youth groups such as Scouts and Cadets, it was then implemented amongst sports clubs such as junior football teams, and more recently someone has realised that young people learn to fly too. So there you go, a flying instructor should have a CRB.
I say "should" because as has been pointed out in other posts, if you are not working with the afformentioned demographic, it don't see how it can apply.
My main problem with CRB is that:
a). in my experience it has limited use. I know of one person who had been convicted of assualt who was still cleared through CRB without any further investigation (to be fair to him, he was actually a nice guy and never a threat to children), and another who will not show up through CRB because he has never committed a criminal offence, but is clearly not suitable to be working with children. From the scouting perspective, an old fashoned interview, with the group scout leader is usually good enough to weed out those who are clearly not suitable.
b). in terms of 'child protection' (read adult protection), at a very basic level you should never be left in charge of a child without the presence of a second adult. The theory being that an innocent adult is open to unfounded allegations if he/she is in sole charge of a vulnerable person. Two seat trainers? Go figure. And it's okay saying that it would be impossible for an individual to commit an offence in flight, but that may not necessarily stop an allegation from being made. In the days of trial by media, allegations alone are extremely damaging to an individual, convicted or not.
In such an instance would the presence of a CRB be useful in defending yourself? No, because all the CRB does is state that a person has not yet been convicted of committing an offence.
The CRB serves the employer. If an allegation is made against an instructor, they can say 'Well, he/she seemed like such an upstanding pillar of his/her community and we did do a CRB check and all was fine, we couldn't possibley have known this was going to happen'. The instructor meanwhile, is still facing the allegations.
So realistically it comes down to this, if you are willing to take the risk and teach students under the age of 18 (and it would be a shame to bar keen individuals from learning to fly), then get a CRB. However, I will (if the system still requires me to fill out another) be asking my employer to cough up the funds, because it really only benefits the company .
Also, Hotinfo: 12 years olds is another thing altogether. Please correct me if I am wrong, but the minimum age for commencing flight instruction is 14, and therefore children under that age can't do trial flights.