Someone posted to the PUPTCRIT mailing list, requesting a safe, easy and inexpensive way to produce "smoke" for the transformation scene in a production of "The Frog Prince". 9-11 year olds would be producing the smoke and watching the production.
Instead of a smoke effect, why not use a flash effect...such as a flash camera. Being an elementary school teacher, myself, I would question the use of a fire extinguisher. Also talcum powder isn't recommended for babies even.....corn starch is recommended and has been for at least 24 years when my son was born! It is dangerous to inhale the stuff. The corn starch and turkey baster idea is probably the safest, but could you really get the kids to clean up the mess??????? I have ALWAYS had problems with getting kids to clean up their messes after the completion of the project/event/production. I would suggest you consider this before picking your effect.
I've always used CO2 fire extinguishers. It is a huge exciting blast of a white cloud, which easily hides the switch and looks like real magic. They are available for rent or purchase, and in small and large sizes. As far as safety goes, you could mount the nozzle in place so it could only be pointing in the desired direction, and avoid any accidental blasts in someones face. CO2 is cold and won't be a problem in a short blast, but if it was shot on someone's hand for a long time it might be a problem I guess. I've used it for years without problems, but not with kids doing it. That's my 2 cents. Axtell Expressions Inc.
I agree more with Steve's suggestion of CO2 fire extinguishers--also what we use--I've heard several objections about baby powder, both talc and corn starch varieties as being quite dangerous for those with any breathing problems, i/e asthma--also can make floors quite slippery--a variation on the co2 method is to attach a long tube--like the s fabric rolls on, or even wrapping paper, to the existing nozzle of the fire extinguisher--this way you can get the smoke up and out, away from the puppeteer's hands--alos shows more to the audience--depending on the dryness/dampness of the space, you'll need to test out how long a blast you'll need, and how quickly it dissipates--but does work well. The other option is the Rosco or chemical foggers, but they take longer to produce the smoke, and do have a chemical aspect.
One Way Street Puppets offers "Smoke in a Can" that works really great. It's an aerosol type spray & you don't need to use much to get the great effect that you're looking for. It runs about $18 per can but if used right, the can will last quite a while.