I don't thing your 9yo will need an NI number. Here's the info I found....
"How to get an NI number
If you don't already have a NI number you must apply for one:
as soon as you start work
as soon as you or your partner claims benefit
To be able to apply you must be:
over 16 years of age
resident in Great Britain (England, Wales or Scotland)
You apply through your local Jobcentre Plus, Jobcentre or social security office who will arrange an interview with you.
If you are a parent or guardian and receiving Child Benefit, any children you care for will automatically get a card showing their NI number just before they reach the age of 16."
(I got this off the
www.direct.gov.uk site....)