Actually, you can (I assume you use Family Sharing). Either you or your kids Steam client needs to go "offline" and you'll be able to play games from same collection simultaneously.
I don't believe you can, as of a few weeks ago in any case.
When you drop offline, any shared library you had access to will drop off. When changing connectivity during a session, your games terminate within 5 minutes.
When you drop offline, any shared library you had access to will drop off. When changing connectivity during a session, your games terminate within 5 minutes.