12 is a bit on the young side for our games, but if you were to pick one, Assassin's Deadly Game would be our choice. The puzzles are more varied so the chances of finding one that fits their style/skill level is higher. We do have a policy that if you bring in kids under 13 you will have to book the whole room so no strangers are added, or book a game on Wednesday which are all private.
I think it would depend on the twelve year olds & which room you picked. My team picked the Secrets of Niribu, and there was 8 of us. It took all 8 of us to figure out all the different clues to finish in just under 55 minutes. I'm not sure how hard their other rooms are, but I would think you'd need more than 2 people to finish within an hour.
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