A few more snippets of text:
So... I'd say that pretty firmly puts the child in the legal status of "whatever they were beforehand, that's what they are (legally), until they are apprenticed."
So the question then becomes, "can you claim loss of magical power not from what they ARE, but from what they WILL be?" Potential earnings, rather than Current Value, as it were. All claims of loss of magical power we see listed in the book (and we have seen them several places) have been from loosing an apprentice, a magical device, or a familiar. There were no examples of a Gifted servant. In this case, I would rule that the burden of proof is on the claimant who is trying to expand the franchise.
Again - I could easily see this as part of the Peripheral code, as Thebes seems to have done. But the Gernicus writeup makes no mention of it in the discussion of the Code as it relates to Apprentices. The Apprentices book makes no mention of it in the "before Apprenticeship" section, or in the "perils of delaying the Opening" section, and the Core Rulebook makes no mention of it in the Apprentices section.
More generally - Laws are restrictive: it is assumed that someone can do anything, except that which is expressly prohibited. I don't think anyone is saying that someone couldn't CLAIM a loss of magical power due to the loss of a Gifted servant - but in all the places where it would reasonably be discussed, it is never brought up as part of the process - again, from Apprentices:
So - here we learn that a Gifted servant can be considered magical power: in their ability to add to a lab total. But in the one section where the rules discusses the legal and social consequences of "what can a magus do when their Gifted servant is taken from them?" - nowhere does it say "sue them for loss of magical power." And it explicitly states that taking that Gifted lab assistant as an apprentice legally covers the new Paren from legal consequences of that "depriving the magus of magical power".
So we have established that a Gifted Lab assistant is, in fact, part of a magi's magical power, in that they are part of his Lab Total. So, if one magus were to steal a lab assistant, and use them only for a lab assistant (or simply kill the lab assistant in the process, or out of spite, or whatever), then yes: there is cause there for claiming a loss of magical power. However, the core rulebook states that keeping a Gifted child as only a lab assistant is one of the explicit REASONS that the Order allows for kidnapping and taking of apprentices - to prevent such activities from occurring.
And nowhere in that text does it say "oh, and the magi who rescued the Lab Assistant must pay for it".