CRB checks are not just for treating children, but for treating the vulnerable as well. Normally massage therapists are not required to be CRB checked. There is no reason why you can't treat the under 18s without a check, but you would require a chaperone who is either a parent of gardian. Although even if you are CRB checked, it would be wise to have a chaperone present when treating the under 18s.
That said, your insurance may have other limitations to who you can and can't treat. But as Energylz said it's best to check with your Professional Association first.
HTH,
Myarka