Hello, I'm not sure where that quote is from. I don't think it's from this article of mine.

If that quote is accurate, I believe it means that minuscule amount of spike proteins were detectable up to 15 days on average. One person had detectable spike proteins on day 29, one day after the second dose, so this person most likely got the second dose on day 28. The spike protein disappeared two days later, so I believe the spike proteins are no longer detectable on day 31.

You raised a good point about weight difference. Honestly, I don't have an answer to that as there's no study about it as far as I know. If I may assume, I don't think it'd make much of a difference compared to adults. A relative increase (e.g., 10-20%) to an already minuscule amount will still be a very minuscule amount. Recall that the spike protein amount that the Harvard study detected is 100,000-times lower (100,000 x 100 = 10,000,000%) than the amount that 'might' cause harm.