Now I realise that many parents have younger children too, so can only come to watch the event with the little ones in tow.
And I realise that little ones do scream and shout a bit, and there's nothing wrong with that.
But if they are screaming and shouting when the children are trying to perform, or the Head is trying to welcome the parents and speak; then general good manners and consideration for others and for the pupils means you take them out ... doesn't it?