If they register under their own names (i.e. request their own license key),
and are (obviously) using the computer for home, non-profit purposes (which is the basic requirement), you
can of course install avast Home Edition on their computers.
Also, if there's more than one computer in the house (house more or less meaning a family in the house, i.e. ~individual appartments), they're only required to register once (and can use the received key in all avast installations in that house).
I hope you got the point.
Hope this explains it a bit,
Vlk