I'd go back to around the time my daughter started sleeping through the night. Don't want to risk a reality where my kids don't exist. I'd tell my friend the cancer is coming back, when it's still early enough to do something.
Don't really know what else I'd do. Don't really care. Be nice to live through those years again without changing much else to be totally honest
You should watch the movie About Time, the guy goes back and when he gets to his present again he has a different kid because the sperm is basically random and you can't choose which one makes it. Harrowing stuff.