TBH I haven't come to the strict/conservative conclusion on the water park thing, though maybe I'm wrong to have not done so (I'm not a full trad TBH, not by the standards of this forum, I have trad leanings.)
But I can make sense of a distinction there.
The water park, assuming its a threat, is not an intellectual threat, but an emotional one.
By which I mean: Evolution/Old Earth is an intellectual issue that can be examined and conceivably refuted through intellectual means. You can't do that with lust. The concern with a water park isn't that the person is going to be intellectually persuaded that lust is acceptable, but just that they're going to wind up doing it. Whereas the concern with the planetarium is conceivably that the student might be intellectually persuaded that evolution or old earth is true.
I get it with very young or very impressionable children, but I'm not a fan of sheltering at least older children/teens from intellectual threats. That's not to say you send them off to public school. Public school isn't just refusing to shelter from intellectual threats, its putting them in an environment where they're getting their entire education from secular premises, in other words they aren't being taught how God connects with anything. I went to Patrick Henry College for two years, classical liberal arts school, its a Protestant school (though honestly I know at least like a dozen people who have converted to Catholicism there, in large part due to the influence of classical education, so you know) but despite that deficiency, in a classical Christian education, you are taught how everything, history, literature, science, etc. relates with God and the Christian faith. Obviously I think that sort of thing needs to be more explicitly Catholic, for Catholic children, but you get the point. Subjecting a child to an entire education regime that is hostile to his faith and which disconnects the world from his faith is bad, bad news.
I don't think a one day trip to the planetarium is comparable. *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. And I don't think that that's comparable either to a water park (which while as I mentioned, I might need to develop my convictions on, at least is conceivably an actual temptation rather than a mere intellectual threat) or to a full blown education regime that's contrary to our faith.