Can a 1 time thing happen? Yes. Especially with men who are pretty inexperienced. Almost everyone cheats at least once in their life, regrets it, and learns from it (or doesn't regret it and learns from *that*). Most do this as kids before sex enters the picture, or as adults while they're still single. In every single case where a married person has cheated, that I know of, it falls into one of 2 camps:
1) Either they cheat with intent, regardless of how their marriage is (from where the old saying comes from "There are 2 kinds of men... those who cheat and those who lie about it"). Those who cheat with intent can still feel massive remorse, but there is a big difference between *intentionally* seeking to have sex with someone, and ...
2) Or they cheat on accident. There was no intent. This can either be an OMG-Sh** one night stand or develop into an affair that they don't know how to end. (This is the inexperienced group.)
Your husband cheated on purpose. Seeking someone out on Craigslist is full on intent. Completely premeditated. Specifically during a time where you were grieving, which is especially low. He may or may not have learned from it, he may or may not STILL be cheating (men and women frequently "test the waters" by owning up to something they regard as a "lesser" offense). Regardless, he handed you the ball when he told you. He either did it so he wouldn't have to live with the guilt.
Either way... before you even consider trust... you need to go get a *FULL* STD panel done. Most STD's are curable (although many are also transferable via non-sexual contact... so if you test positive for certain kinds of herpes/warts/etc you'll need to get your 4 year old tested as well. Even HIV is treatable to a certain extent... and you NEED to get on that yesterday if god forbid you test positive.
Ultimately, whether you stay or go... what you can live with, and what degree of happiness you require... is a personal decision.
Best of Luck to you... I've been there.