The rule of thumb last year, was 15 minutes before park closing.
It's best to do the fireworks then the 2nd Fantasmic. It is possible to do the early F! and then fireworks, but it isn't very easy to find a spot where she could see them. People complained last year of their children not being able to see them after F! because of the street already filled with people and children too small. Best viewing for little ones is around Echo lake and Minn and Bill's. There was an area near the Idol screen where everyone sat, so it was easy to watch A&E on the big screen and see the fireworks between the now gone hat and the "Idol" marquee.