You certainly won't be guaranteed seats together, and neither should you be. The rest of us have to pay so why shouldn't you? The airline should ensure that children are sat within one row of a responsible adult. Not sure how TCX works but at some airlines a 16 year old can be a responsible adult for a 14 year old so they can sit together.
Bear in mind that every day children as young as five fly across the states completely unaccompanied. It is not that big a deal.
You have the option to sort the issue out now but you are consciously choosing to take a chance so don't kick up a fuss if you can't get seats together. It's the summer holidays and lots of families will be taking the gamble at you are and ere will always be losers unfortunately.