What if you found someone you trust (close friend or family member) to watch your son? Then would you be able to go along as a chaperone?
If you really can't do that and you do decide to keep her home (and yes, I am a strong believer in trusting your gut instinct!), then maybe you could do a "nature thing" with her (and your son) near home -since that's what the group is doing.
For example, you could write up a little scavenger hunt for her, you know; "find two leaves, a rock you find interesting, 2 flowers of different colors, etc" and then collect those things in a paper bag. Then, bring those items home and talk about them (describe them using your senses, etc) - maybe even do a little research on those items online or something like that. Or make her do a little project about them. She could draw a picture of each item, and then under each write a few words describing it...
I bet she'd love that at her age - AND she'd get the one-on-one attention (well, one-on-two in your case ;)) that she will not get at the field trip. That way she won't feel like she "missed out", she just got to do "something different" with mommy and brother. Something special just for her :)
Whatever you decide to do, good luck!!