The meetings are just for the staff, right? Kids won't be there?
I'm in two minds about this. I do agree with everybody who says it's ridiculous, and to threaten legal action if they have a problem with you not dressing up. It's the principle of the thing.
On the other hand, it's just for a bit of fun, isn't it? I admit the warning that you would 'regret it' was far too heavy-handed. Is dressing up at a childcare centre really so ridiculous? Is this a time where you really need to put your foot down? If your colleagues aren't getting into the spirit, there's bound to be a little friction there, just as there might be with someone who never gets into the spirit at office parties.
I hear so often around TFP that you should 'pick your battles' but I guess I'm unsure whether this is one you need to fight.
Then again, like others have pointed out in threads like this, they can't get mad at you for not dressing up now. But they can be less leniant on you about other things, and get you that way.
EDIT: Cynthetiq - I see you wrote that while I was writing this
