I'm not so sure it's wise to allow an 18 year old who,has left the nest for whatever reason to come back into the nest.
I'm at a loss to understand why you'd bring this up since the original post makes no mention of the possibility of the child coming back to live with the OP. However, the notion that a parent would give consideration to landlord-tenant issues over the parent-child relationship is rather absurd.
in CA a tenant has one super pack of rights....and if adult child smells like a tenant it now becomes a Herculean task to remove them and might even cost you a bigmwad of move out money.
Not sure where you're getting this. In a situation like this, California law isn't much different than most states, and there's nothing even remotely "Herculean" about serving a 30-day notice and filing a UD complaint. I'm also not sure why you think it might cost "a bigmwad of move out money" (whatever exactly that might mean).