Don’t wait. I think you should either fill out a new schedule A with the right info and send (via webform) or send out a webform with the additional info you forgot to add (The USA visa denial) stating you forgot to add that. My sisters friend got denied and banned for five years for misrepresentation because he forgot to disclose the time he was denied a visa to America. Even after explaining himself. His case however was not family class. He was trying to get a tourist visa. From what I’ve read about misrepresentation cases, the trick with CIC is contact them before they contact you. That way, they believe it was actually a mistake and you weren’t trying to pull a fast one. Usually, when they notice the error and send out that “procedural fairness” email stating you might be banned for misrepresentation, the chances of getting your visa even after explaining yourself is usually very slim because most times “I forgot” doesn’t cut it for them. They might not be so strict in the case of family class, but better safe than sorry..
PS: my sister made the same mistake when her family applied for express entry. Once she realized, she sent a webform explaining her mistake and a couple months later they got their visas.
