H2O2 can cause eye damage. When used with reasonable care, it is unlikely to cause anything but temporary skin bleaching - if kept away from the eyes. That and maybe some ruined clothing (or other surfaces). I'd not use 30% around children, although as a controlled demonstration -physically separated from the audience, it can be safely used. First thing you could do, depending on your resources, is to determine if your 3% is actually near 3%. There are tables which list its density, if you can weigh some out into a volumetric flask, say. If you have a chemical refrigerator, it should be stored cold (and of course, away from sunlight). I guess if it were me, I'd buy some fresh 30% and dilute it for the kids to use (prior to the session). I'd use potassium iodide rather than yeast (what is it that you're trying to teach?) and possibly consider FeCl3 (if the lesson is about catalysts) - although staining would be more of an issue with the iron salt. Make sure that the peroxide isn't phosphate stabilized (if possible). If you really want to use yeast, make sure it's fresh, and I'd give it at least a minute or two in 35°C (95°F, which is just warm, not hot) water. Try different brands. Also consider your liquid dishwashing soap - they're not all the same. Some sources suggest adding glycerol to stabilize the bubbles - I've not tried that, but it seems reasonable (my old bubble recipe calls for 0.5 cups DW soap and 2 Tablespoons glycerine...). No reason you shouldn't add the KI to the soap (technically, it's a detergent) - you'd have to futz around to see if any water was needed and if so how much. At 3%, that means (do the math) that 100 ml will generate ~2 liters of O2 (at room temperature) - so a 20X total expansion maximum (ignoring volume increase due to steam formation from exothermicity). Figure the reaction is only partially complete and that figure drops. Seems to me the important thing is mixing - how the two solutions are added to one another. If it were me, I'd add the soap to the peroxide...although if I was leading a bunch of kids maybe I'd use them to try out different techniques. Proper eye protection is, imho, eye goggles - not safety glasses. This experiment is energetic and uncontrolled, use caution (with anything higher than 3%-6% peroxide).