When you get to the gate go right up to the agent and explain the situation about not sitting together. I am 99% sure they will take care of it right then, especially knowing it is a wish trip. And even if they don't there will definitely be a few nice people who will understand and be willing to switch with you. Don't stress about it, I have seen them fix things like this many times, even for me and my DH without any kids with us, they have worked it out.