It really varies. There are quite a few laws regarding as to what is a firearm, replica firearm and airsoft firearm.
There is a useful thread here: http://www.airsoftcanada.com/showthread.php?t=23034
It's long, but it will give the detailed info about airsoft when it comes to legal issues.
Eg. For instance, if one of the kids modifies their shooter to fire at 500+ FPS, he will be in a world of shit if discovered, since by law anything firing at that velocity may be considered a firearm.