Yeah, I agree with MusicianDad. I wouldn't put any reason why you won't be there. A simple "I am unable to attend" is much more polite. If they follow up or ask why, you can explain your reasons then.
I also wouldn't go.
I would be fine with your reasoning. I wouldn't want my first grader to miss school that early in the year. They're usually going over rules and routines that early on and I wouldn't want him to miss out and then later find out he was...
We're going to start in a couple weeks.
Nancy Larson Science
Myths Maps and Marvels for History
Writing With Ease
All About Spelling
I'll also sometimes do stuff from the Learn at Home...