A few things to add to the great list below, or to reiterate because I think they're important:
Meet with the teacher in the next few weeks to go over her expectations, needs, recommendations, and calendar. I agree about 3-4 stations of small numbers of kids, vs. 25 kids doing the same activity. Put together what you think is a good program, then delete one thing - you'll find you're all set, really, given the delays and some glitches!
Find out rules about costumes (although that ship has sailed for this year), things like Valentines (and I assume Valentines is like Halloween, that there are some religions that don't permit it), and food allergy policies.
Instead of emailing individual parents or getting them to sign up manually at some event, use SignUp Genius. It's so easy and you will find it's better for managing people or updating your needs as you go along. It also sends reminders on whatever schedule you dictate, and people can see what others are doing which either gives them inspiration or makes them reluctant to bail out because their life got busy. Those who cannot pay may be able to help out in person, which is great. Get to know your fellow parents to be sure you know what their skills and abilities are, as well as their shortcomings. (Those who always come late, or those who like to boss the kids around should be relegated to some other task.) This also lets you recruit a helper parent - lots of people won't sign up to be the main organizer, but can be terrific assistants.
Do allow for financial contributions for those who don't have time off, and do allow for those who are financially able to kick in a little extra to help out a family in need (you don't have to name that family!). Any leftover money can go to the next event.
If no costumes were allowed for Halloween, consider something like Crazy Dress Day or Upside Down Day (kids wear clothes inside out, wear pajamas, mismatched socks, etc.) but not associated with any particular holiday. To encourage reading (even in pre-readers), consider a "Dress as your Favorite Character" Day.
Anything you send out should have the wording and spelling checked by another set of eyes. (For example, you asked for "advise" but what you meant was "advice" - for some people, that really counts when you are representing an educational facility.) And double check your instructions and quantities and due dates and event times - it's amazing what looks fine to us when we've read it 20 times, but which is confusing to the recipient. It's so hard to undo it and send out corrections. And when you tell people a time to report or bring their stuff, add 15 minutes! If you need them and their stuff there at 11 AM, tell them 10:45!
Good luck!