We have to remember too that Harry was in Estonia as a representative of the Queen. When William and Kate visit NY and DC, they will be visiting as representatives of the Queen (they always are I think) but mostly they're there to represent their charities and causes. It will not be like William's visit coming up to China where he's primarily representing his monarch. I do think its a big honor that William will meet with the president and vice president and its a reflection, I think, of the recognition of the work that he is doing for illegal poaching and trade.
On the subject of being the heir to the heir, I think we need to remember that William is the odd duck in this respect. Most other 3rd in line to the thrones are still children.
I loved reading the story about the little boy wanting to take Kate home to play with Thomas the Train. I'm willing to bet my last glass of eggnog that Kate will be very well informed and be able to carry of quite an intelligent conversation with him about Thomas. Perhaps George has a Thomas the Train too.