Hm. I'd say that the apprentice certainly counts as property, so it falls under the "I stole a book" rule. However, the utility of such an act is questionable, and if abused will likely result in a convenient Bonisagus showing up and deciding that "I really should have that apprentice, and oh no I have too many apprentices and whoever can take this new apprentice from me, oh looks the original magi has space for one, can you foster this apprentice for me, thanks." - and the Tribunal will, absent any additional evidence, back them up on that.
And I also agree that familiar appears more complex...except that in thinking about it more, I don't think it is. "Destruction of one's familiar" is a known punishment to a magi. As such, that suggests that the familiar, like the apprentice, is considered legally linked to the magi. In contrast, for example, the Order probably doesn't declare "destruction of one's Wife" or "death of one's child" as a suitable punishment.
(Although that may be prevented by the various interpretations of "don't mess with mundanes" - as killing someone's family to punish them may bring in the inquiry from the Church and/or secular authorities, who tend to frown on anyone but themselves doing that sort of thing. Wizards killing wizards? Fine, whatever - but making declarations about the deaths of people who aren't wizards? Whoah, there!)
However - no one wants their familiar kidnapped. So - I would argue it as "technically permissible, but will probably get the rest of the tribunal to declare Wizard's War on you, just to prevent Familiar kidnapping from ever becoming a thing." - which would likely in turn get a ruling at Tribunal, and everyone agreeing that it will not be an OK thing to do.
So, "Technically permissible until someone does it, at which point the Tribunal votes to make it illegal." I agree with Silveroak on that, anyway - it's a short term solution until Tribunal.
EDIT - however, a contrary argument is the "if you can't kidnap it, we may as well just kill it" argument - which would probably fall into the Rule of Unintended Consequences.