Select the right helpers. Older tween or young teen actors are best for this exercise; they can read better than middle school-age members of your company, and will consequently give the sample scenes more "life." This will give you--and them--a better idea of the overall quality of the script and how the play will flow as a whole. Avoid choosing actors with personality or ego issues; this is work, not a performance.
Make the first cut of scripts in advance. Choosing from 50 possible plays is overwhelming. If your theater season is three to five plays long, select 10 or 12 strong candidates in advance. The pre-chosen scripts should represent a variety of story genres such as comedy, folklore or mythology, musical, and drama.
Mark the sample scenes beforehand. Select a scene or two from each play that has enough dialogue and simple action to engage your young actors and help them get a feel for the work as a whole. Photocopy (and shred immediately after to comply with copyright law) enough copies of the scene for each character in the scene plus a few extras so you and other critiquing adults can read along.
Find a large space, like one you would use for rehearsal, and meet there. A play is meant to be seen as well as heard, and reading sample scenes in a theater setting will give you a glimpse of how the play will work as a staged performance. Have stools or chairs off to the side of the staging area for adult judges, including yourself, and young actors who may not be in a specific scene.
Rate the scenes. Create a rating sheet for each play's scene in advance. Determine criteria--pace, believability of dialogue, characterization, and overall story quality, and make a rating system so all present can "grade" each scene. Select the plays whose scenes ranked highest for your next children's theater season.