I think it's fair enough, albeit a shame for those who would like a full GF menu.
I can't stand badly behaved children in restaurants of any kind and have been very strict with my own 2. The one time DS1 misbehaved in a restaurant was mostly because there were 3 other small children just allowed to run riot - under tables, around tables, racing each other - and DS1 was only 3 and couldn't understand why he couldn't do it if they could. It was in the restaurant of the hotel we were staying at, so DH (who had finished his meal) took DS back to the room with the rest of his dinner so he was no longer distracted by the ferals, and I finished my dinner in solitary state before rejoining them.
I think the reason for banning under 12s (rather than a younger age limit) is because it also frees them up from having a "children's menu", as the cut off age for those is usually 12.
I don't really blame the pub for this. A restaurant local to where I live very nearly banned children because of a couple of feral incidents - but luckily for us, they remembered that there were decently behaved kids around, so decided not to. They didn't allow the feral families to book again though!
I had a sneaky thought, but I doubt it would be allowed - since patrons are allowed to put up reviews of the restaurants etc. that they've been to, wouldn't it be handy if the restaurateurs were able to put up reviews of the awful customers! I saw that many places already share info on runners, aggressive customers and ferals - but it would be great if there was a "Customer advisor" review page too. Pretty sure it wouldn't be allowed under data protection laws though.