Out of curiosity, I called today to ask about kids and the buffet breakfast. It sounds like they don't have the prices written down... they couldn't tell me anything precise, the amounts I got were, respectively, $19 and "around $20." That is, for our family of four, as Diamonds, I understand that we could upgrade 2 people from continental to buffet ($19 ea) and the 2 kids (6-12) would pay the same or about the same. The concierge did indicate that children over 12 are treated like adults for buffet purposes.
To be clear, I asked the front desk and then the concierge. I didn't ask to be transferred to the restaurant... if there is any fixed practice about this, I guess they would be the ones to know it. Maybe betonic will tell us... or maybe there is no actual answer to these questions, it's just what the waitress decides to charge.