I'm forever transferring tracks off a flashdrive onto the kids PC then taking the drive away and the kids saying 'Dad! The songs aren't working'.

Simply by importing you are just telling iTunes where to look for the track. Take the drive away and it cannot read them.

Create a folder in your music folder and copy the actual data for the tracks over THEN tell iTunes to look in that folder. You should be able to Copy entire folders across that way, but it might take time if you have a lot of tracks. You won't be able to play the music unless it is actually physically on your laptop.

ITunes is just an 'organiser', it does not actually hold the music data itself.