that if you drop off your child to see a 12a cert film with their mates, that they should all be 12 or over

[SNoraWotzThat]

and not to find that some have taken their younger siblings along, who are clearly 8 years old.

12a - If you are under 12 you must be with an adult 18 years or over.

What's unreasonable about expecting people to adhere to the rules set out by the BBFC? As somebody has pointed out, if the group were caught with an underage child, the lot of them could have been thrown out, therefore denying the original poster's child the chance to see the film.