I remain glad that I told my kids. To this day, however, my wife doesn't know I told S-then-14; if she ever finds that out, she'll likely go ballistic, but I didn't want him to EVER think I would lie to him, and I wanted him to know the reason why our marriage was failing, if it was to fail.
As is the case of most affairs and a child of his age . . . he already knew, and had been horribly confused and upset.
