There was a time I remember (not so long ago...!), when those children who were not very outgoing or those who were 'unreliable' were given 'the instruments' to play in the school nativity play! Pretty outrageous, if you think about it! Firstly, what does that say about playing instruments? Secondly, how boring it must be to play a triangle throughout the show when you’d rather be a beautiful angel or a wise man. And then to be expected not to fiddle with your tambourine in a quiet part of the dialogue!
Rather than have a ‘dedicated band’ playing instruments throughout your show, let different children play instruments at different times throughout the show, so that they have a bit of variety in their roles. Have an ‘instrument area’, perhaps a large soft mat, where instruments are laid out ready to collect and play, and where they can be replaced gently afterwards. Different groups of children can come to the area to play the instruments in one particular song and then return to their ‘home base’.