Despite being tall and well built for his age, her 14-year-old son has just passed the 60kg cut-off mark for under-10 rugby. But watching her son, at under-15 level, tackle one of the opposition's bigger players was "like watching a Mini taking on a bus".
Mrs McNab believes the situation is leading to more injuries and, in particular, concussions in young players.
The Herald's analysis of rugby-related ACC claims found a huge spike in injuries reported by 15 and 16-year-olds in 2013. ACC received 8611 injury claims from a playing pool of 18,098 15 and 16-year-olds - close to one claim for every two players throughout the country.
Mrs McNab said a decline in playing numbers among 15 and 16-year-olds could be directly linked to the injuries and lack of enjoyment created by mismatches.
"Every year we lose more players because of the size mismatch and, every year, it is a little less safe for the ones who stay in the game. Eventually you have to bite the bullet and say, 'This can't go on. We have to do something about it'."
Since May, she has written repeatedly to the Wairarapa-Bush union, New Zealand Rugby, college principals and the Ministry of Education without receiving a satisfactory response.
She now plans to take her campaign to schools' boards of trustees, questioning their duty of care.
NZ Rugby community rugby manager Brent Anderson said the issue was on the agenda at a meeting of provincial delegates next month. "We are very aware and conscious of the issues that are out there. But, as many parents who write to us worried about big kids playing against little kids, we have an equal number of people saying, 'My big kid hasn't got a team to play in.' How fair is that?
"It is complex - there is no right answer - but it is in everyone's interest to make the game as safe as we can and that is what we look to do."
Wairarapa-Bush Rugby chief executive Tony Hargood said the union was focused on providing an enjoyable playing environment for as many children as possible, but did not compromise on player safety.
"Our ultimate aim is have young players, boys and girls, enjoying their Saturday morning rugby or Rippa and we try and put a lot of emphasis on participation," Mr Hargood said.
"Mixing teams from all areas - bear in mind we are a very rural region - provides challenges."
Meanwhile, Sir Brian is not involved in his daughter's campaign, but he does support her aims.
He agrees children are being placed in danger and being scared away from the game.
"I do believe it is an issue all over New Zealand," he said. "I've been along to a little bit of kids rugby to watch grandchildren. You'll get one or two big kids who, instead of running into a gap, will just go running into these little fellas because it's neat knocking them arse over head. That is the bit that really upsets the parents."
Mrs McNab did not mention her father's name when she approached the Herald with her concerns.