There are some pieces available for brass and children's choir, but they are generally for a larger brass group and also include an adult choir.
Cantus Natalis - by Z. Randall Stroop, has a beautiful Children's choir part.
Best bet, find a great children's choir piece, and re-write it for your group. Don't get too lavish on the orchestration, as you will drown out kids' voices at a less-than-robust mezzoforte.