if you want to do a short one-act at the beginning of the show, or at the intermission of the show, try playscripts.com. i'm not sure if they have anything related to "meet me in st louis", but a lot of the casts are expandable, there are shows as short as 10 minutes. There are usually excerpts you can read as well, to get a feel for the story. Be aware, though, some of the shows on there can be very strange.

 

anyways, the director at the middle school where i'm a "student director" (a group of HS students come back and help out) is always trying to add in parts for more kids. If you know how long your theater has been around for, you can do a retrospective of the history of the theater. IE have one or two kids assigned to each year, and have them prepare a 1-2 minute speech (they can be longer if its a newer theater group) about the show performed in that year, maybe some social context, etc. We did it as a preshow onstage, and kids could tell their families to show up however many minutes before the show to watch them. If your group has been around long enough that such a program would get tedious, you could try having the kids do a similar thing in the lobby of the theater, so as the audience enters they can stop by and watch a few presentations without being forced to sit through an hour or more of the presentations. 

 

other ways to include more kids is to have some dress up in costume and usher, or have an expanded ensemble which only participates in 1 or 2 songs, which could dance in the aisle. you wouldn't need to call them to as many rehearsals, and if your music director and choreographer are up for it, that could add a bunch of kids to your cast.

 

anyways, sorry that was so long, and good luck with the show!!