I ask a lot of my assistants. The days go by easy but they might be long or just start early or end late.
Yes, they have to have insurance to use the gear outside the studio but other than not working ourselves, they have complete run of things.
I do expect questions and queries as I want our working environment to be a learning one. I don't mind them using the studio for free and when the bigger jobs come along, we both understand that things will change and costs and rentals will have to be paid for, but in the meantime, please, test and work away.