Most laptops have two microphones at the top of the screen -these will be used by default if another one is not plugged in.  They often good quality but prone to picking up background noise.  You can plug a USB or jack mic in which will allow children to "close mic", put the mic just below or to the side of their mouth so that it picks up their voice better in noisy surroundings.  

If you use a plug in mic, you will need to check that Audacity is using that mic (referred to USB or Jack mic) and not stuck with the built one (array mic).

Try and prevent children from holding the mic as this will introduce noises from their hand.

• When you save as a PROJECT, the file will have a little Audacity icon.  It can only be loaded into Audacity and will come up with all the tracks that you have been working on.  Always save work as a PROJECT if you will want to come back to to edit and add to it.  It's also a good idea to regularly save it whilst working so that work is not lost if there is a crash. You can use [ctrl]+s to save quickly.

• When editing is finished, then it should be EXPORTED as an mp3 file.  This merges all the tracks to make one file that can be played in media players.