Hi abbey71211,
Fantasmic is an amazing show that is a definite MUST SEE! I've been to DL countless of times and I make sure to see Fantasmic each and every time without fail.
There is no queue for Fantasmic - there's no line to line up for. It's held at the Rivers of America so it's "open seating" - you seat/stand anyplace they will allow you to seat/stand.
If you want a premium spot from where to watch Fantasmic, you will need to allot at least 45 minutes to stake out your spot. If you're with a large group, you can have some members of your party save the spot with blankets while one adult escorts the kids to nearby rides (ex. Pirates of the Caribbean, Splash Mountain, Haunted Mansion comes to mind). I wouldn't recommend leaving only one person to stake out your territory while everyone runs off though because soon enough people will start inching into your territory, blanket or no blanket.
Premium Fantasmic viewing spot would be someplace that allows you direct and unobstructed view of Tom Sawyer's cabin across the River of America. Keept in mind the pavement between the veranda and the ropes is designated as "seating only." This means everyone who occupies a spot in this area must seat down, regardless of whether you are child or adult.
When in doubt, ask a cast member who's working the Fantasmic crowd. They're usually happy to give you tips.
Hope this helps.
P.S. On a May Thursday, there is usually only one Fantasmic show.