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", she said.
Mrs McNab believes the situation is leading to more injuries, and in particular concussions in young players.
Sandra McNab believes New Zealand Rugby has abdicated its responsibility to ensure players are kept out of harm's way. Photo / Lynda Feringa
The Herald's analysis of rugby-related ACC claims revealed a huge spike in injuries reported by 15- and 16-year-olds in 2013 (see graphs below). 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.
Player numbers by age groups
Injury rates by age groups
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 local schools' boards of trustees, questioning their duty of care.
NZ Rugby community rugby manager Brent Anderson said the issue would be on the agenda at a national meeting of provincial union delegates next month.
"We are very aware and conscious of the issues that are out there," Mr Anderson said.
"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, both boys and girls, enjoying their Saturday morning rugby or Rippa and we try a 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."
Support from Sir Brian
Sir Brian, a former all Blacks captain, selector and manager and current New Zealand Rugby patron, is not involved in his daughter's campaign but he does support her aims.
"I support her concerns, obviously, along with a lot of other parents."
Mrs McNab is petitioning New Zealand Rugby to introduce and enforce better controls governing the size of players following a string of incidents resulting in nasty injuries to young players in the Wairarapa.
Sir Brian said he agreed with his daughter that children were being placed in danger and 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 little kids' rugby to watch grandchildren. You'll get one or two big kids who, instead of running into the gap, will just go running into these little fellas because it is neat knocking them arse over head. That is the bit that really upsets the parents, I am sure."
Mrs McNab did not mention her father's identity when she approached the Herald with her concerns.
New Zealand Rugby's general manager of community rugby, Brent Anderson, a former All Black who earned 69 caps for Wairarapa-Bush, said rugby administrators faced a difficult job in balancing the needs of all children who wanted to play the game.
"The bigger unions with a big playing population can structure their grades to mitigate against big kids playing against small kids," Mr Anderson said.