Some Children's Theater companies will only mount shows for which they have collected costumes, sets and props for years. They just rotate around the same shows utilizing resources they already have. That's about the only way they can manage to make a small profit from their programs year to year.
School and workshop productions usually require the children to wear the Workshop T-shirt and shorts, or to provide their own "costumes." Even with coaching as to what to provide, the costumes will not look very coordinated. Some shows are much easier to costume than others, and director's take that into consideration when choosing the vehicle. If they have to make a choice between spending money on costumes or providing the many other things needed for the show, they will often opt for getting the other things.
For example, I was able one year to buy many dark blue/green and pink peasant skirts from some thrift stores. They have been used in many shows for different purposes. In "Parizade's Quest" they were the Persian dancing girls' skirts. In "The Adventures of Dick Whittington," they were the kitchen maids' skirts. Different tops and an added scarf, apron or hat changed their look.
I have collected black knit pants and solid colored tops for years --- in many different sizes. Black pants and T-shirts with added colorful tutu or shorts was the basic costume for "The Ants and the Grasshopper." Black pants with a solid colored top became the children's basic costumes in "A Successor to the Throne."