I agree with everyone else who told you that you should never take a gig that you don't have a show prepared for. I've been working on a new kids show for the past month and it won't be ready until at least the end of February.
Seventy kids will be tough if you haven't done a kids show before. Knowing the ages will allow me to give you better advice. Also, knowing how long you need to perform would be helpful.
The best advice is to go with what you know. Now isn't a time to learn new effects. With that in mind, there are some effects you can pick up pretty quickly. The patter will be pretty much "look at me" but there really isn't time to do any better.
Mark Wilson's Tic Tac Toe Using a White Board (PM me if you want some tips on presentation)
Vanishing Bandana
Invisible Deck Using a Beach Ball and a Paper Bag (I'm pretty sure I've written this up before so you can find it using the search feature)
Sponge Ball Blendo - The basic routine is self-working. If you have any sleight of hand skills, you can incorporate a moe complicated spongeball routine into it to make it longer.
How to Make a Zebra - OK, you can Google this and you won't find anything useful. Get a change bag, a 9 inch black silk, a 9 inch white silk and a two 9 inch zebra silks (or one 18 inch zebra silk). You can get smaller silks and a zebra streamer and do this with using a TT. Use your imagination for the patter. Ask what color is a zebra. Have them cheer for white and then for black. You get the idea.
One last comment. Don't fall into the novice trap of performing trick after trick with no presentation. Pick effects and pair them with presentation so that each effect will be around 5 minutes.
Feel free to post what other effects you have and I'm willing to help you develop something resembling a show with some good patter.
