CRB checks can take a while, and can hold up the application process for new adult staff, or when a CI goes into uniform either as an Adult SNCO or VR(T) Officer.
However, the wording of ATC regulations on the matter does not - if you read between the lines - preclude new applicants (e.g. CIs) from service whilst their CRB/application is pending, it just says that they must not have UNSUPERVISED access to the cadets. For example, very few Civilian Committee members will be CRB'd (I would imagine), but will do welfare jobs such as running the Sqn canteen etc. however, in doing so, they are not given unsupervised access.
If an applicant is happy to work "escorted" until their CRB/official appointment comes back, there is no problem (although this needs to be made clear to them). For example, I have 2 CIs - both brothers, and ex-RAF - on my Sqn. One brother joined before the other, and was cleared by the time his brother joined up. Therefore, they worked together until the other brothers' CRB came through, so that his access to the cadets was not unsupervised. Incidentally, this worked rather well, as he was able to gain some valuable instructional technique training by observing his brother in action...