for our end of y2 play we hold auditions. we all get together in the hall - all y2 teachers and TAs in the hall, with all 90 children. Those who want a speaking part get lines to practise for 10 minutes. Then the stand up (in groups of about 8-10) in front of the staff and other children and say their learned lines. Staff will then write comments and give positive words to the child (but never giving anything away re parts) and on it goes. If it is a play with solo singing, or small group singing, we audition singing parts too. Its all done in good nature and in a fun way and so far every year it has been a good afternoon.
Staff then get together and work out who should have what part...we never pay any attention to parents and their roles within school (or not) and it is always about the child - how we think they will cope, how confident they might be, if we think they can learn the lines or sing a song, etc. Everyone gets to do something - but we do take into account any who really don't want to talk on their own or dance, etc.
Once parts are out we won't change them - regardless of parents, etc.
We try and make auditions a fun day and then we make sure everyone in the year gets a proper named part and one where everyone gets a chance on the stage, plus a role that matches their preferences and their capabilities on stage.
Another thing we do is to split main roles - so half way through the show, the main cast change people. Works really well.