I would only use this for a gag rather than a serious disappearing mic trick, but each to his own. A pretty versatile gimmick at a good price!

I like the foam microphone. It looks pretty real and sometimes I like to fool around a bit with it before the show.
Try to speak throuhg the microfone and getting some interaction with the kids whether it is working or not and why not.
Then after a while just throw it in a paper bag or something and crumble it and throw it away ("cause it is not working anyway")
A bit like you can make the neilsen bottles dissapear but more with a ;) because it is no problem if the audience figures out it was not a real mic.