RTI & Under 16's

A client of mine who employs waitresses who are 15 has asked me about including their pay in his RTI submissions. Whilst I believe under 16's can usually be excluded from the FPS on the grounds that they aren't liable to NIC's and can't claim benefits, HMRC make it clear that they CAN be, even if they don't have an NI number. The following is a paste from the HMRC web site:

'Whilst HMRC wants employers to provide correct National Insurance numbers for their employers in their PAYE submissions wherever possible, there will be occasions where a National Insurance number isn't available (for instance, when an employee is under 16 years old).
In these cases the guidance in 'Making sure you use the correct National Insurance number' is clear that 'You must leave the National Insurance number field blank for that employee. You must not use an incorrect or 'dummy' National Insurance number.'
What is essential is that, when a National Insurance number forms part of a real time PAYE submission, it is correct.'

The comment I have had from my client however, who wanted to process all staff through payroll, was that OpenPayroll wouldn't let him save an employee with a blank NI number. I haven't tested this myself, but if he's correct and if this isn't an RTI requirement, should it be enforced in the payroll software as it appears to be?