Overall, I would agree that, whether or not RJ wrote this into the culture, I would imagine that testing there is a traumatizing effect on children. Not just girls--boys are tested too, except that male channelers are killed, IIRC. And men tend to exhibit the spark later than women, so their terror could potentially last a lot longer.

So, there are a couple of things to consider, although there are a lot of details we can't be sure of. First, channeling is a rare talent. So, most girls will not be collared. It's a genetic lottery, and given that channelers never marry or have children, the genetic tendencies are rare.

However, on the flip side, having a sul'dam in the family is a great honor, and is far more common (although it too is rare--just less rare than being a channeler). These are folks born with the ability to learn, but not channel innately, which I would imagine is on the genetic 'channeling spectrum'. Now, we don't know how often sul'dam marry. Very often? Do they tend to have many children? Are their lives subtly lengthened because of their quasi-channeling?

Are there no farmers in Seanchan? Does no one understand breeding? The fact that Egwene figures out that sul'dam can be collared, and yet NO Seanchan person in the culture itself has figured this out is frankly amazing. It defies belief, but RJ seems to suggest it is so. These folks really are brainwashed by their society into not thinking certain things. It's actually incredible that they've managed to maintain an empire for so long and conquer so much of Randland so successfully, given that they have been trained from birth by their culture to be so blinkered about what is staring them in the face. Tuon, who despite being rather xenophobic and superstitious, is actually pretty sharp, and yet even she hardly blinks an eye when finding out her culture is built on such extraordinary misdirection.

Perhaps the super-secret-society of Seanchan High Elders knows this, and would have informed Tuon when she became empress, had she not already known.

Anyway, back to your question. In sum, seeing as Aes Sedai rarely marry or have children, and have been 'culling' male channelers as well, and seeing as this has led to a decrease in the number of channelers, it stands to reason that channeling ability is rare in Seanchan as well. Only about 1 in 1000 is collared, and the chances go down from there after age 20, even if they don't let up until 25. The marriage issue is a good one--I wouldn't put it past Seanchan culture to kill the babies of women who later find out they can channel, or else turn them into da'covale for life.