Take a small dairy drink bottle (such as that of DanActive/Actimel), wrap with aluminum foil so that the bottle is wholly covered, and about 4" (10cm) of foil are left at the bottom.
Tighten the remaining foil to create the microphone's cord. Use a tooth pick to puncture small holes in the part of the foil covering the bottle's opening, to create a microphone look, but mainly - to allow it to function!
Indeed, since the bottle is a small sounding board, when speaking very close to it - it will amplify your voice. Very slightly, and yet - a real microphone, home-made by the kids!