Janet, that sounds wonderful. I hope you have a great time there. I can't advise you on what to wear, because we don't have anything comparable here. My older three did get to have lunch with the Governor General a few years ago (when is was Sir William Deane - I wanted to go too and meet him, but they couldn't fit me in as well). They went as Young Carers, because of the help they gave me with my disability plus their baby brother. And yes, you can be a difficult child and still a Young Carer.
When the kids went to GG lunch, they wore smart casual clothes. There was a lot of playing around on the lawn. The media were there too - be aware, yours will certainly be a media opportunity. So no rude fingers at your 'favourite' congressman! There are kiddies watching...
Marg