Also, yes, it's known that girls (and their older male counterparts, lol) like Precure. But, I doubt that they'd keep sub-18 year olds out for fears of such encounters.

I guess Japan has finally ditched the pretense that shows like Precure are for little girls and that the target audience is not actually 30-year-old men. At the upcoming Precure All-Stars DX 2 event, according to Canned Dogs, they're going to be doing an all-night screening of a bunch of the movies.
The thing is? 18+ only. Even if the kids have a guardian to go with them, they still can't go.
I can only guess what the crowd is going to look like at this sort of event, and I'm sure it's understandable not allowing kids to go to these events. There'll probably be an impromptu doujinshi swap or such like that.
Either way, if you're in Japan on April 3rd, then you may as well shuffle over and give it a shot. You can make some friends for life, talking about your mutual interest in the deep plot of the show. That is why you guys watch this stuff, right?