Hey there Lincol, While I'm not quite sure how the Arena does it there are a few ways of doing all ages gigs; most all ages gigs are just a case of proving ones age before a drink can be purchased at the bar, this is by a coloured wrist-band that is given to the drinker upon entry to the venue with an 18+ ID. There is another way of doing it where a big black X is placed on the hand of an underage person upon entry. Having said that, if an underaged person wants to drink, he/she will find a way (either lax staff or a mate purchasing the booze for them, or frauding the system) and the most that can be done is the authorities imposing heavy penalties on those responsible.
About the mingling business, the mixing of ages usually does happen because keeping separation is usually expensive in smaller venues, but there is no way to tell until you ask the organisers about it.
I hope that helps a bit with your question, and I hope your kid has a safe gig. (Laurei)