12 means 12
Just because a film is classified as a 12A, it does not mean bring along your 8-year-old child and use the cinema as a babysitting service.
The BBFC were somewhat taken aback when people started doing precisely that, following the revision of the 12 certificate to 12A. 12 still means 12, unless you think your child is especially capable of dealing with scenes of the court of Henry VIII engaging in incest, or James Bond being hit in the genitals with a knotted rope.
"In my absence, there was a big news story with a lot of people writing in to complain to the BBFC for giving The Dark Knight a 12 certificate, and the way their complaints went was usually this: "I took my eight-year-old to see The Dark Knight and they were really upset"; "I don't think this movie is suitable for 10-year-olds"; "I took my 11-year-old to go and see - " you know what, it's a 12. It's a 12 certificate. The clue's in the number there. What the BBFC think is, is that the movie is suitable for 12-year-olds and up. And there's a big notice at the beginning of the film that says, "it's a 12 certificate movie. If you're bringing somebody under the age of 12 to see it, it's your responsibility." I cannot believe the amount of rubbish that's been written in the papers allowing parents to complain about taking under 12s to see The Dark Knight and then being surprised when it upsets them. It's a 12! That's the way it works! A 12 certificate means - take a running guess at this - you probably should be 12 in order to see it! And if you take someone who's not 12 to see it, it's your responsibility. I'm not making this up - there's a sign that comes on at the beginning and says the accompanying parent takes responsibility for it!"
While Mark did take his not-yet-12 kids to see the Zac Efron film 17 Again, he did so having read the BBFC advice and acting as a responsible parent, so that is alright and not a case of being nothing if not inconsistent.