bummer all around.
We have/had a very sick daughter too (she 2.5 now too) Luckily it was never anything "horrible" just countless minor things with her. She's had a few hip surgeries, tonsils and adenoids removed too. If its any hope for you, while the post tonsil/adenoid stuff SUCKED HORRIBLY, she did get better and really hasn't been nearly as miserable as she was pre surgery either. IT really did help her from getting colds and sinus infections etc... so, while it does suck now, it will get better.
As for the cancer, that just blows. Hopefully it was all caught early, and while nothing related to cancer is "good news", the good news is that it is so treatable nowadays its amazing. NOt saying tough times aren't ahead, but life (expeciually with children) is never going to be easy.
One day at a time, ejjoy the love you have for your children and wife, and their love for you. That is why we are here... there is no other reason. Best of luck, you have my thoughts and prayers and, no matter how hard it is, have a merry Christmas and a happy and blessed New Year!