I will make the suggestion that your problem here is not the instrumentation, but the low skill level of the instrumentalists. Arranging for low-level band is a complete art and specialization in and of itself.
Unless every one of your players is a prodigy, there is likely to be a very limited set of rhythms, and even notes that you will be able write for your ensemble. You may want to ask the help of the band teacher to loan you some primary school-level scores so you can start to get an idea for this (they should also be able to tell you where in the curriculum the students are, so you will know what notes and rhythms you can use).
If your piano player is an adult, and you have an existing choir/piano score to work from, your job is made MUCH easier since you can basically copy over the voice and piano parts as is and spend your entire effort working on the other instruments. If you only have a lead sheet to work from, your first effort should still be to generate a score that can stand on its own with piano and voice -- the other instruments should then lay on top of this groundwork (the adult pianist will be the musical glue that holds everything together if the choir or instruments start to go off the rails). If your pianist is also a beginner, then you have a pretty challenging task on your hands.
Assuming that is not the case, your directive then should be to keep things VERY simple, and don't get in the way of the voices. Group the instruments into only a few different parts (2 or 3) and then keep those parts closely related to the piano/voice material. If you have an instrumental part playing with the choir, it should probably be quieter instruments matching the melody in unison or occasional close harmony. (This can enhance the choir instead of clashing with it.) If you have spaces between vocal phrases with a countermelody in the piano, this would be a good time to use other instruments. Still another example would be an underlying background part, perhaps a rhythmic one, that would last throughout.
So, to summarize: keep it simple, the singers are the important part except for when they're not singing, write to the specific skill level of the ensemble.
There are existing published works out there for young choir with instruments -- perhaps check out http://www.jwpepper.com/ to try to find some examples.