Thank you Jandm.
Looks like INZ is checking mainly for "risk factors for tuberculosis (TB)" using the x ray submitted.
I have taken an appointment for the parents for the x ray for June 15th as the x ray report should be less than 3 months old. I'm planning to apply for extension end of June 2018.
I had applied online for the visitor visa. Today when I checked online, I did not see any option to do an extension, but only to apply for a new visa and then provide the previous application number, and together with a covering letter detailing the reason for extending. I hope this works for now.