Leeneia:

It will be a bit harder to accomodate the kids who bring band instruments to your session than you realize, because they will need their parts to be completely written out, and they also need to be conducted, and they will need the other musicians and singers to exactly follow the the conductor as well--

As to the parts, in addition to being transposed to an appropriate key--it is important to know that a number of the band/orchestral instruments typically do not play the melody line, and so these parts have to be created and written out as well--the long and the short of it is that, in order to make it work, you'd really have to do all the things you'd need to do if you were starting a beginner's orchestra and chorus for the kids, starting with the purchase or comission of written arrangements

The best arrangement(least trouble, most fun) is to simply have the kids sing along with an informal guitar or piano accompaniment, and save the band instruments for band class.

I