i think at this age group you can start setting more definite positions.  you will still find some kids can be played anywhere, and some you are still trying to work out where they fit best, but mostly, i would be setting them into more solid positions as it will help with team coesion.  they need to start working as a solid well oiled team by the end, and if you are still letting everyone play anywhere, then that will never happen. but i guess its up to the division of the team too.  if they are just coming for fun, then set positions really dont matter, but if we are talking competitive netball, then start working out who plays where.