D.W.
First of all, I hope your prognosis is good and treatment is going well.
I was diagnosed with Hodgkins on 6/10/08 and often speak about how cancer changed my life for the better. I hope your experience will, too, and that you and your husband had an incredible, long life together!
As for thanking your families. I don't think it's necessary to do anything expensive. What they've done, they chose to do because they love you both and wanted to give you something amazing to celebrate your decision to be married.
What cancer taught me is how important the people in our lives are. If possible, I'd take them out to lunch at a nice place (as a group), have a dinner at your houses as a thank you, send each of them a picture from the wedding in a nice frame with a special note, etc.
I believe strongly in sentimentality now after cancer touched our lives. My diagnosis was right after having our second child, so I understand how it affects you with a young child as well.
I'd write a heart-felt letter, inscribe something wonderful into a book that reminds you of them, put a small photo album of favorite pictures of you and them over the years.
At the end of the day, we want to be connected with people, and memories are usually the best way to celebrate the people who touch our lives so dearly.