i see what you're saying, but I really don't find it nearly as jarring (or jarring at all). But that's just me.

As to frequency, once you'd pointed it out it did feel frequent to me, but I wanted to go back and count to eliminate confirmation bias. Counting all applications of "young" or 'child" with relation to jack and Clare, I got four times this chapter, four times last chapter, three times each in chapters two and three, once in chapter one -- but in chapters three and earlier, it was usually not stressing their youth, usually just using "young man" or "young woman" as a basic descriptor.

So it has been more frequent, and more paternalistic, in the last two chapters. Not sure how it compares to other adjectives and descriptors.