The poll didn't include the option, "I wouldn't let my children go to an SSPX school."

I would have in the 1990s; definitely wouldn't since 2012.  

What mushy liberalism!
"A child would be hurt if he could not join the group."
The reality is exactly the opposite: A child would be kept from harm if he did not join the group.
Moreover, group/herd-mentality is hardly the breeding ground for the very critical and independent thinking you want to advocate.
Ever been to St. Mary's?
The rationale that deliberately placing our children into occasions of sin (e.g., proximate dangers to the faith) is OK, and according to this person, even meritorious, is completely delusional.  
The presence of a father does not remove the danger to the faith.
Should I be able to take my child to an immodest waterpark, on the grounds that I will be there to explain to him about Catholic modesty?  Will that arm him against the inroads of the devil, or prevent against the onset of concupiscence?
There seems to be some confusion here between occasions of sin which are deliberate (e.g., going to a planetarium; waterpark), and occasions to sin which are necessary, accidental, and unavoidable.
Only in these latter, accidental/unavoidable occasions to sin is the post-facto talk about correcting the errors of the presenters permissible.
To deliberately enter into an occasion against the faith, then talk about it later is not morally permissible.
If it were, what is the basis for not going to the Novus Ordo?  
Why, you could just explain to the family what was wrong after the fact, while you plan to attend again next week!

*Even if* the SSPX wouldn't correct errors, that you feel they should correct, you can still talk to them about what they learned and teach them the truth.

I can't say for sure because there are some variables but one thing to consider with your point of view is that when parents homeschool, their children are more accustomed to having their parents teach them like this. Go through the lessons at home, get to the truth and the proofs, then take them out and show them the errors of the world once they are ready. When children go to school however and especially an SSPX school where they revere their priests and teachers, the parents might be at a disadvantage in trying to contradict said teachers.

I personally know of big disputes among families where the parents were taught one thing by the original SSPX but their children or grandchildren are being taught something else by nice SSPX. The parents usually lose out. They taught their kids to listen to SSPX priests, so the kids just think -- well this is what my SSPX priest told me, so it's fine.  

Parents have to set themselves up as their children's primary teachers from a young age and establish a good, trusting, teaching relationship with their children if they hope to be able to correct anything. Otherwise, children will see their other teachers as the experts and will tend to think their parents are wrong.  

It's easy to say just correct the errors but the real question is how likely that child is to take their parents' word over their teachers'.