In my experience I´m agree using songs in class with groups of 10 years at the beginnig, middle or at the end because it´s relaxing for the students, I can use like warm up or like a review, sometimes they don´t like, but they are producing the language and they remember some vocabulary or functions that they learned in class and sometimes they relax.
Yes, I prefer start the lesson with a review because the students starts to focus with the class, I have their attention.
No, I don´t make an extra material for those students that finish first, but for the material that I prepared for everybody I explain it to those, and they keep working.