I spoke to the guy at CRB today and he said though not required it is advisable to get a certificate to shoot kids. He echoed the rules about self employed not being able to certify themselves and it haveing to be agencies or employee's...etc.
ALSO...he mentioned another certificate which can give peace of mind to clients. Its called a 'subject access check' You get it via the data protection guy at the county police HQ to where you live. It costs a tenner and you'll get a certificate at the end. Can take between 30 to 40 days to go through.