No, it is not automatic.
I have 2 stepdaughters I have only "met" via FB and MSN video calls. There are plenty of applicants with stepchildren in Canada they will never meet until they have PR because the applicants will not be given TRVs and the sponsor/parent does not have permission to take the children out of the country. So, no, not everyone has met their sponsor's children.
The forms have to envision situations like the one I mention above, so that is why the questions are phrased that way.
It makes sense if the sponsor has children from a previous relationship, in that case I agree with you that chances are the spouse being sponsored may not have met those children yet.
So, shouldn't question 15.A read instead "Do you have any children
from a previous relationship? If yes, list them below". Then, question 15.C would make complete sense.
Is my logic flawed? so, how am I to answer Questions 15.A and 15.C?