Just a single unit of 12. More like a combat patrol. Just allows him to get used to unit interaction, how a Commander can join squads, how cover and cover save bonuses work, how different weapons on the same unit work together and how to pick the right unit for the right job.
My mate is making a similarly balanced SW list. The lads brother will be taking control of those for the game. They will be playing against each other and we will explain what they need to do, then let them decide how they want to go about it.
They are only about 7 and 10 I think, bit young to be here, but their dad seems interested as well.