Are kids sneaking into R-rated movies still common?

I heard a rumor that kids bought tickets for the Wild Robot and went to go see Terrifier 3. How common is it for kids to sneak into movies today? Before it was common, and it happened because when you purchased a ticket, you didn't also select your seat, so it was kind of free for all, but now you order your tickets and then you have to select your seats, so I would assume it's not very easy for kids to sneak into R-rated movies.