What you need to look out for is Cryptosporidium. It is highly resistant to chlorine.
The CDC recomends
(
http://www.cdc.gov/healthywater/pdf/swi ... ridium.pdf)
15,300 Contact Time of chlorine to kill Crypto.
That is FC X Minutes = 15,300
In the example they provide, FC of 20ppm would require 765 minutes (12.75 hours).
Here's the bad news: This chlorine level is in unstabalized water.
A footnote in the link states:
This level of Crypto inactivation cannot be reached in the presence of 50 ppm chlorine stabilizer, even after 24 hours at 40 ppm free chlorine, pH 6.5, and a temperature of 77°F (25°C). Extrapolation of these data suggest it would take approximately 30 hours to kill 99.9% of Crypto in the presence of 50 ppm or less cyanuric acid, 40 ppm free chlorine, pH 6.5, and a temperature of 77°F (25°C) or higher.
And, the presense of this bacteria is not going to register as a FC drop. (First, the FC may not be killing it, and if it is, there isn't really enough to register).
On the other hand... some public pools and water parks have several POUNDS of fecal matter deposited on a daily basis just from inproper cleaning. This is a great reason to have our own pools.
Being you know the source of the exposure, I would just make sure the kid isn't sick, and go back to swimming after a really good 24 hour shock.