First of all, it's VERY ok. You have to be strong so much of the time, and press on regardless fo whatever comes your way... because that's the nature of life. So if you have a few minutes for YOU, and even if it's a little pity party, I say take it!
As far as the trip, here's my opinion (albeit from a childless girl!). When it was just the school, I thought forget them. You will have a very legitimate reason for going, and a doctors recommendation to support it. The kids won't miss much as far as material, and the trip sounded perfect.
With the information and concerns about Rees, though, I think I'm going to be the oddball and say I would understand more if you went in December. I really think that it will be "ok" either way, and you will all rebound and adjust. I just think that the extra time off and not throwing the schedule off too much sounds more beneficil, IMO.
Keep coming back as long as you need to vent and or switch back and forth; I know none of us mind, and we all want you to make the decision that brings you the most peace!
ETA: apparently while I was typing I became less of an oddball. Well, I wouldn't go that far

but more people agreed with the December trip than when I started posting!