.How early do you have to get there before Fantasmic begins?
Depends.....you should be OK about 1 hour before, but you may not get the very best seats. We went 1 hour before on the first Monday in October (thinking it wouldn't be busy) and were seated in the far left section. It was standing room only at 7:00pm. Our seats were OK, but the best is in the middle between the two Mickey flags.
Do you recommend the meal plan so that you get preferred seating?
If you want to ensure you get seats, but you still have to go about 45 minutes prior.
Also, how intense is it for little ones....can you get up and leave if necessary?
Yes you can get up and leave. How instense, really depends on your child. DS saw it the first time when he just turned 4. Keep in mind that Splash Mountain and Haunted Mansion was no problem for him. He freaked at Fantasmic!!! He thought the Villians were going to hurt Mickey, and I could not convince him otherwise. We almost had to leave! In October DS now 5 1/2, still started to freak during the show. He started to cry and wanted to leave, but I was able to convince him that it was just "make believe" and Mickey was still OK. So, it really depends on your child