Hi Chris
You would have to ask the reg. office about the ones not showing on their index, some may not be completely indexed online, the website should show what coverage there is
If you manage to find out the cemetery and grave numbers for the right children, (and whether there are headstones) from Middlesbrough cemeteries office, we can go and look for them and take a photograph for you ( bereavementservices [at] middlesbrough.gov.uk )
Barbara