I feel like the rules such as "keiki menu is for guests under the age of 9" are there to protect themselves from the cheapskate family who goes to a nice place like 'Ama 'Ama and orders a round of ice water, shares an entree among a few people, has others order off the keiku menu, and stays for a long time taking legitimate business away from the restaurant and tip income away from the server.
If you, on the other hand, have a normal meal (e.g. order some drinks, every adult gets their own meal, etc.) and your 11 year-old wants to order a kid's meal, I don't think they'll have any problem with it.